A federal judge ruled yesterday that a shareholder lawsuit filed against Baltimore Bancorp because it rebuffed a takeover bid by First Maryland Bancorp last year can go forward, but he dismissed portions of the suit naming 19 corporate officers and board members as co-defendants.

Judge Marvin J. Garbis also threw out part of the suit naming the investment banking firm of Alex. Brown & Sons as a co-defendant.

Baltimore Bancorp and First Maryland waged a bitter six-month battle that ended last Nov. 1 with First Maryland withdrawing its $17-a-share offer for Baltimore Bancorp. That caused Baltimore Bancorp's stock to drop from about $10.50 a share to about $5.

In a class-action suit filed in November in U.S. District Court in Baltimore, Frank Tischler, a shareholder from Baltimore County, accused the Bank of Baltimore's parent company of engaging in fraudulent behavior to mask the fact that its stock was worth substantially less than $17. Officers and board members of the $3.3 billion company were accused in the suit of "acting individually and in concert" to suppress unfavorable information. Alex. Brown was named as a co-defendant because it gave financial advice to Baltimore Bancorp.

The suit seeks unspecified monetary damages on behalf of anyone who bought common shares of Baltimore Bancorp stock from May 16 to Nov. 1, 1990. Mr. Tischler bought 500 shares of common stock on Aug. 31 for $9.50 a share. The stock closed at $8.875 yesterday.

The attorney representing Mr. Tischler, Charles J. Piven, said he would proceed with his client's suit. "If additional information comes to light with regard to other defendants, a decision would have to be made at that time whether to refile against those defendants," Mr. Piven said.

Regarding the dismissed portions of the suit, one of the attorneys representing Baltimore Bancorp, Wilbur D. Preston Jr., said: "I'm delighted, and I think the judge did exactly what he should have done."