In a painful new twist to the insider-trading investigation on Wall Street, Lazard Freres & Co. said yesterday that a young former financial analyst is under investigation for divulging inside information about mergers to his "big brother" at the firm.

The "big brother," Robert M. Wilkis, 37, a former Lazard Freres investment banker, was a confederate of Dennis B. Levine in the largest insider-trading ring ever uncovered, the Securities and Exchange Commission has charged.

Lazard Freres said yesterday that it has received an SEC subpoena asking for information about the activities of Randall D. Cecola, 25, who worked at the firm for nearly two years, until July 1985. The subpoena is connected with the government's investigation of insider-trading cases involving Levine and Wilkis, the Lazard statement said.

"Bob Wilkis apparently befriended him. . . . and became a big brother to him, and then exploited that relationship" by allegedly getting Cecola to talk about deals that should have been kept confidential, said one source.

Cecola's attorney, Alan Kaufman, said yesterday, "I don't know him to be under investigation. The fact that he received an SEC subpoena doesn't mean that he is. . . . I don't see that he's got any insider-trading problems. If the impression that people draw is that Randy Cecola was part of the Levine-Wilkis insider-trading ring, that's very unfortunate for Mr. Cecola."

Levine, a former investment banker who made $12.6 million in illegal profits through investments based on confidential information about upcoming takeover bids and other deals, has pleaded guilty to criminal charges. He is awaiting sentencing and is cooperating with the expanding investigation of "Wall Street's Watergate."

Wilkis, who moved from Lazard to E. F. Hutton & Co. before being implicated with Levine, has agreed to forfeit $3.3 million in illegal insider-trading profits and other assets. A court order bars him from future work in the securities industry and criminal charges against him are anticipated, according to the U.S. Attorney's office in Manhattan.

Cecola, from Barrington, Ill., joined Lazard after graduating from Purdue University. At Lazard, he was highly regarded and he had the firm's encouragement when he left in 1985 to seek a master's degree at the Harvard Business School, sources said. Having completed the first year at the school, Cecola is working in Manhattan as a summer associate at the investment banking firm Dillon, Read & Co. He could not be reached for comment.

Lazard received an SEC subpoena concerning Cecola's activities on Friday. Early this week, Cecola voluntarily went to Lazard officials to describe his involvement with Wilkis, according to one version of events.

A Lazard spokesman declined to elaborate on why the firm issued the statement yesterday.

Meanwhile, the U.S. Attorney's office in Manhattan released a letter late yesterday outlining some of the evidence it has turned over to attorneys for a defendant in another insider-trading investigation.

The letter identifies three Wall Street executives whose telephone records were furnished to the defense attorney; they include Irwin Schloss, a principal of Marcus Schloss & Co., an arbitrage house.

Until this spring, Schloss' firm employed Andrew Solomon, 27, as an analyst in its arbitrage business -- the short-term trading in securities of companies involved in mergers or takeovers.

Solomon pleaded guilty on June 5 to being part of an insider-trading ring that allegedly relied on information from a 27-year-old attorney, Michael David, who had worked for a firm specializing in corporate law.

Solomon told the court that he had passed on information from David to two superiors at Marcus Schloss, but did not name them. Solomon is to be sentended on Friday. David, who has pleaded not guilty, is awaiting trial.

A letter from Assistant U.S. Attorney Kenneth I. Schacter to David's lawyers, filed recently in federal court, outlined the evidence the government has provided the lawyers.

That evidence includes telephone records of Irwin Schloss and members of two other New York arbitrage firms, Spear, Leeds & Kellogg, and Edward A. Viner & Co. Inc., as well as David and another defendant in the case, Robert Salsbury. Schacter declined yesterday to comment on why the telephone records were mentioned in the letter, and an attorney for Schloss also declined to comment. New Market Detection Plans Reuter

Officials of the New York and American stock exchanges and the over-the-counter market said yesterday they had developed sophisticated new methods, including computer programs, to spot insider trading.

"For the first time, we have been able to bring masses of relevant data together in an easily retrievable and flexible form so investigative and enforcement authorities can do a faster, more comprehensive and effective job," New York Stock Exchange Chairman John Phelan told a House Commerce subcommittee hearing here.