Boston's lawyers pride themselves on the work they do for free. It's a way for them to give back, and it's a major recruiting tool. But as top firms cast an increasingly critical eye on the line between public service and profit, some fear good will could end up lost in the balance.

Bingham McCutchen LLP readily boasts of the important legal work it does for free: petitions on behalf of Guantanamo Bay detainees, death penalty appeals, support for women's reproductive rights.

But for Bingham and Boston's other largest law firms, balancing pro bono legal services with money-making work is a challenge. Retroactive to Dec. 1, according to a memo obtained by The Globe, Bingham soon will require any lawyer who approaches 150 hours a year of pro bono work to meet with a supervisor to discuss the ''scope" of those activities.

Senior partners at Bingham said the new guideline, which will create a uniform pro bono policy following a merger, is a management ''flag" meant not to discourage pro bono work, but to ensure that its lawyers are involved in projects that further their professional development.

''These check-ins are not going to be some big sit-down with someone yelling at you," said Nora C. Cregan, chairwoman of the Bingham committee helping to craft the new policy. ''We're going to say, 'Tell me how this case is going. Do you need help? Where do you expect to go from here? Are you getting good experience?' "

Still, some associates at the firm, as well as lawyers elsewhere, said the guideline could curb pro bono activities by suggesting a 150-hour cap on work done by highly compensated attorneys that by definition doesn't directly bring in revenue.

''There's a constant tension, no question," said David L. Ferrera, a partner at Nutter McClennen & Fish LLP who co-chairs a pro bono committee for the Boston Bar Association. The bar supports the Supreme Judicial Court's ''aspirational goal" that Massachusetts lawyers work at least 25 pro bono hours a year.

''We're a for-profit entity, and we're able to do pro bono work because we're in business," Ferrera said. ''But no one wants to be flagged if you're an associate. It sends a message to attorneys, subliminal or otherwise, that after some given amount of time pro bono clients are less worthy of your work than paying clients."

Pro bono, which in Latin means ''for the good," has long been a proud tradition of the legal profession. It is rooted in a belief that because lawyers are often among society's most highly paid and skilled professionals, they should give back to the community. The principle is reflected in a widely circulated cartoon in which one lawyer says to another: ''Remember, we can only afford to do all this pro bono work because of how much the anti-bono pays."

Pro bono programs are also major recruiting tools, used to attract clients and lure law school graduates.

At most firms, both junior and senior lawyers do pro bono work, but young associates in particular tend to cherish it because it taps the idealism that drew some of them to the profession. It also teaches valuable legal skills they may be unlikely to get from paying clients early in their careers, such as taking depositions and arguing appeals.

Oddly enough, pro bono work, which usually must be approved by a firm before a lawyer takes it on, can also boost the pay of the lawyers who do it. At most firms, billable hours are primarily how lawyers earn their keep, and generally refer to hours billed to paying clients as well as hours spent working for nonpaying clients.

At Bingham and many other big firms, associates are expected to bill around 2,000 hours a year to be eligible for annual bonuses, and pro bono hours can often count toward that goal.

The new policy at Bingham -- which handles local legal matters for The Boston Globe -- is still among the most liberal, allowing unlimited hourly credit for approved pro bono projects. Still, three Bingham associates said it could put a chill on pro bono work but wouldn't allow their names or comments to be used, saying they were concerned about workplace consequences.

The memo describing Bingham's planned policy tries to anticipate such a reaction: ''It is critical that folks understand that the intent of this sit down is not to set a 'bar' or otherwise to deter attorneys from performing pro bono hours; rather, its intent is merely to discuss with the attorney the scope of the pro bono activities s/he has been performing . . . and/or whether it is appropriate for the firm to get other attorneys involved in a given matter."

''It's a delicate balance, because it's important to have senior attorneys checking in to make sure pro bono representation is useful," said Robin A. Dumas, a former Bingham lawyer who said she billed 400 to 500 pro bono hours a year when she was there. Dumas left last year to become general counsel for one of her former pro bono clients, Partners In Health, which still receives pro bono services from Bingham.

''But people at a large firm where there's a lot of pressure to bill could interpret that message in a negative light or as the point where you're yanked off a case," Dumas said. ''So the challenge for Bingham's management is how it's going to play out in practice."

The American Bar Association does not require its members to do pro bono work, but says all lawyers have a ''professional responsibility" to provide at least 50 pro bono hours a year.

Large law firms nationwide have increased their pro bono activities in recent years, even among firms that did not increase headcounts and despite competitive pressures that have spurred a trend toward consolidation, according to Esther F. Lardent, president of the Pro Bono Institute, a Washington, D.C., group that encourages firms to devote 3 or 5 percent of their total hours to pro bono.

Bingham's new policy, which is still being drafted, attempts to meld separate pro bono policies that have existed since the 2002 merger of the Boston firm Bingham Dana and San Francisco's McCutchen, Doyle, Brown & Enersen LLP.

Bingham placed no limit on pro bono credit. McCutchen gave unlimited credit only for what it called ''impact" pro bono work, meaning high-intensity cases that often require high numbers of hours.

Senior partners at Bingham, which has grown in recent years to nearly 1,000 lawyers, said the 150-hour marker is a tool to better manage firmwide pro bono activity.

They noted that lawyers are already asked to touch base with a superior on their pro bono work, and that work for paying clients is similarly reviewed.

They also said the average Bingham lawyer did 85 hours of pro bono work in 2005, up from 47 hours in 2003 -- well below the 150 hours that would cause pro bono work to be flagged.

Neil McGaraghan, a seventh-year Bingham associate who said pro bono work accounted for 565 of his 2,215 billable hours last year, said he believes the new policy will be constructive.

''I actually think the message that it's good to sit down and evaluate whether or not your pro bono workload is furthering your professional development is pretty positive," McGaraghan said