The messenger in the blue jacket riding the elevator at the New York Stock Exchange sported a button on his lapel that asked, "Who's Next?"

The button was a beer maker's promotion, but it asked an explosive question as this financial capital faced the largest federal probe of insider trading in history. On Thursday, Wall Street merger specialist Dennis B. Levine and four indicted defendants pleaded guilty to federal charges stemming from two separate investigations of insider trading. A fifth indicted defendant pleaded not guilty.

The defendants have pledged to cooperate with prosecutors. They presumably are naming others who shared access to an illegal grapevine of confidential information about mergers and takeovers that was used to advance their careers or pad their bank accounts.

Who will be the next to be implicated? Only the government investigators can say.

Three New York financial firms have acknowledged that they made internal investigations to see whether the grapevine had run through their offices.

Levine told U.S. District Judge Gerard L. Goettel that an unnamed source gave him the tip about a planned takeover bid by Chicago Pacific Corp. for Textron Inc. in 1984 that enabled Levine to make about $200,000 by buying Textron securities before the offer was announced.

Lazard Freres & Co., a leading merchant banking firm, disclosed Thursday that it is reviewing its records to see whether it was a source of the tips Levine received on Textron and other takeovers.

Levine never worked for Lazard Freres, but the firm was an adviser on 10 of the nearly 50 transactions cited by the Securities and Exchange Commission in which Levine allegedly made $12.6 million.

A Lazard Freres spokesman said its inquiry did not turn up evidence linking anyone "presently employed" as a source of leaks to Levine.

Lazard Freres prides itself on its professional discretion. Newcomers to the firm are warned that leaking information "is the only mistake they can only make once." Lazard Freres' staff includes several of Levine's former colleagues at Shearson-Lehman Bros. Inc., where Levine worked from 1981 to February 1985.

"There is no question, everybody is asking, 'Did he name somebody here?' " the Lazard spokesman said today. "All we know is that no one associated with the firm has done anything to arouse our suspicion."

Marcus Schloss & Co. Inc., a risk arbitrage firm and New York Stock Exchange specialist, also has conducted an internal investigation.

Andrew Solomon, an analyst at Marcus Schloss until March, pleaded guilty Thursday, admitting he had shared inside information about takeovers with several friends. Solomon told Goettel that he passed the information on to two unnamed principals of Marcus Schloss, and the firm "used the information" to purchase securities, allegedly knowing the source of the information.

Irving Schloss, president of the firm, issued a statement saying that, if Solomon ever acted on inside information, he did so "on his own, without the knowledge of the firm. . . . " The firm's inquiry "not only failed to suggest any improprieties," it strongly indicates that there was no trading on nonpublic information, Schloss said.

But Solomon's charges were supported by attorneys for Arthur Ainsberg, a former vice president at Marcus Schloss. According to his attorneys, Ainsberg -- who was responsible for the firm's compliance with trading rules -- learned in March that Solomon was receiving information about takeover bids from Michael David, a lawyer and former codefendant with Solomon who pleaded not guilty to insider-trading charges Thursday.

Ainsberg's lawyers, Melvin Brosterman and Alvin Hellerstein, said Ainsberg observed Solomon discussing takeover information with principals of the firm. He became convinced that the principals were aware that it was inside information that was coming from David, Brosterman said.

Seeking confirmation, Ainsberg invited Solomon to dinner on March 14, asked him about the deals and recorded the answers on a concealed dictation machine, the lawyer said. Ainsberg, who already had alerted the law firm for which David then worked, took the information to the SEC March 25.

Robert Salsbury, who also pleaded guilty Thursday, said he passed tips from David to colleagues at the arbitrage department at Drexel Burnham Lambert and Co. Salsbury, a junior member of the department, said the information was used by Drexel Burnham to purchase stock in Avondale Mills, a takeover target. Drexel Burnham, a leading investment banking and brokerage firm, has said it is "outraged" by what it called a betrayal by Salsbury and Levine, who was a managing director there before the SEC moved against him.

Drexel Burnham has turned over records of its arbitrage department to the SEC, in response to a subpoena.