(FORTUNE Magazine) – Corporate America has seen the enemy, and the enemy is ... a guy named Brad Seligman who runs a nonprofit?

Chances are you've never heard of Seligman or his tiny organization, the Impact Fund. But he's the lawyer who has brought massive class-action sex-discrimination suits against retailers Costco and Wal-Mart (the latter is the largest discrimination case in history).

While plaintiffs lawyers have become emblems for much of what's wrong with the legal system, Seligman insists he's just trying to get companies to treat all employees fairly. He grew up in a well-to-do family in Los Angeles learning about law from his father, who was a prosecutor at the Nuremberg trials. His dad eventually became a TV executive at ABC; in 1969, on the day Seligman graduated from high school, his father died in his sleep from a heart attack. At 51, he had been two weeks away from vesting in his retirement plan. Denied benefits, the family was left to struggle financially, propelling the son on a quest to fight "corporate injustice," as he describes it.

To Seligman, that meant becoming a lawyer. In 1992, while a partner at a small firm in San Francisco, Seligman won a major victory in a sex-discrimination case against a chain of supermarkets called Lucky stores. The $107 million settlement made history (at the time it was the third-largest settlement ever), and the attorney's fees, estimated at as much as $12 million, made Seligman a multimillionaire.

Seligman quit his job and started the Impact Fund, a legal nonprofit devoted to social issues. The fund survives on donations from individuals, foundations, and the California bar. Since opening its doors in 1993, the group has awarded $3.6 million in grants to public-interest lawyers who represent the poor, women, minorities, and environmental issues. (The money must be used to cover costs, not to pay the attorneys.)

Seligman, 53, who recently started collecting a $73,000 salary, could stand to make millions in fees from the Wal-Mart and Costco suits. But he says that any windfall would go back into the fund. "For the Impact Fund it would be a gold mine," says defense attorney John Fox. "We have never seen that side [plaintiffs] so well financed."

Indeed, ensuring the survival of the class action is Seligman's larger goal (winning him no shortage of detractors). Says Wal-Mart spokesperson Mona Williams: "You have a couple of California attorneys who are trying to manipulate the system by rolling out cookie-cutter lawsuits." Costco declined to comment. But Seligman points out that last year a mere 84 class-action suits were filed in federal court, compared with almost 1,200 a year in the mid-1970s. (In the intervening years the Supreme Court has gradually raised the standards for certifying cases.) To help others interested in filing suits, the fund acts as a training ground for attorneys; its annual class-action strategy summit will be held in San Francisco this fall.

Since he filed the Wal-Mart suit, Seligman says his phone has been ringing nonstop. On a recent morning, he said he had already had calls on three interesting new cases. Perhaps the only consolation for big business: "We can't," Seligman says, "sue everyone." --Cora Daniels