When the American Bar Association held a seminar this spring on white-collar crimes, a small group of lawyers sat quietly in the back of the room taking copious notes. They had registered under the names of Smith or Jones and gave no company affiliation.

Unlike the other lawyers there, they asked no questions and talked to none of the participants. They disappeared at the end of each session as quietly as they came in.

They were, in fact, attorneys for large corporations who didn't want anyone to know they had any interest in white-collar crimes.

"They didn't want to admit that their companies faced trouble," said one lawyer who attended the ABA's seminar in Boston.

Prosecutions of corporate crimes are on the increase and white-collar crime has moved from stock frauds and bank embezzlements into the Fortune 500 list of the biggest businesses in the country.

Richard W. Beckler, deputy chief of the fraud section of the Justice Department's criminal section, shocked the ABA meeting when he reported that about 400 corporations - many of them on the Fortune 500 list - faced federal inquiries into possible criminal violations arising from their questionable payments to foreign governments and American politicians.

Those companies had already admitted voluntarily to the Security and Exchange Commission that they had made these payments. But now the Justice Department is looking into possible criminal prosecutions.

As a result, lawyers who had been familiar mainly with corporate boardrooms are now finding it necessary to bone up on criminal law.

"White-collar crime is the fastest growing legal specialty in the United States," said Paul R. Connolly, chairman of the ABA's section on litigation and a Washington lawyer who has defended both politicians and businessmen accused of white-collar crimes.

"All over the country there's a burgeoning interest in how to represent people in the white-collar context," said Judah Best, another Washington lawyer who ran the ABA's seminar.

"Big Wall Street firms," he continued, "realize that instead of referring criminal cases to other lawyers they need someone in the firm who can handle them."

Or, as another Washington lawyer put it, "Firms that would not touch a criminal case with a 10-foot pole now are forced to when chairmen of the boards are being indicted."

The Justice Department has made white-collar crimes one of its top priorities, and according to the FBI, new agents are being specially trained to deal with corporate crimes.

Arthur Lyman, a partner in the New York law firm of Paul, Weiss. Rifkind, Wharton & Garrison, called the area of white-collar crime "a growth industry. The trend toward criminalization of business morality in the wake of Watergate means that the commercial litigant (a lawyer who specializes in trying civil cases) had better develop the skills to handle a criminal defense."

For example, Mark Richard, head of the Justice Department's fraud division, said some corporate attorneys thought their companies freed themselves from federal prosecution by telling the SEC they had made questionable payments.

"They are in for a sad awakening," said Richard. "There's no absolution merely because someone said 'I did it.' At best all you are talking about is mitigation."

This new interest among top law firms in criminal practice means an upgrading of the criminal bar - once considered the drugs of the legal profession.

Thirty years ago, said one lawyer, "the lower end of the bar in talent and ethics" became criminal lawyers.

The bright guy out of law school became a clerk for a federal judge, then spent a couple of years in a U.S. attorney's office. From there he went into a big firm and never tried another criminal case," said Arnold Weiner, a Baltimore lawyer who specializes in defending politicians charged with corruption.

"But the change in the type of person who becomes a defendant has wrought a big change in who becomes a criminal defense lawyer."