WASHINGTON (AP) _ Rep. Barney Frank, D-Mass., said today that he used his congressional immunity to write off delinquent parking tickets for a personal aide who also was a male prostitute, but only when the assistant was on official business for him.

Frank said the aide, Stephen L. Gobie, had used his car while on official business and had access to the car when he was away from Washington.

''There are some occasions when tickets were issued even though the car was being used on official business,'' Frank said in a statement. ''I did ask that such tickets be waived'' by writing to the House sergeant-at-ar ms, he said. ''At all other times, it was my policy to pay for tickets incurred.''

Frank's statement marked an elaboration of comments he made in an interview published in today's editions of The Washington Times.

But the lawmaker branded as ''lies'' Gobie's assertion to the Times that Frank had knowingly intervened to waive tickets the aide received while using Frank's car to keep appointments for illicit sex.

Gobie, who contends he operated an escort service using Frank's car and Capitol Hill apartment, told the newspaper he used the car to transport prostitutes and even to have sex.

''He knew what I was doing,'' said Gobie. ''I used to drive that car around all the time.''

John F. Banzhaf, a George Washington University law professor and legal activist, said today that he has asked Frank to agree to open disiciplinary proceedings that Banzhaf initiated against the congressman with the Massachusetts Board of Bar Overseers.

Banzhaf contends that Frank should be disbarred as a lawyer for the commission of a ''serious crime,'' in this case sodomy. He said the proceedings should be open because Frank has admitted having sex with Gobie and that sodomy is against the law in both Massachusetts and the District of Columbia.

Spokesmen for the Massachusetts board said a state Supreme Court ruling prohibits them from commenting or confirming whether the disciplinary complaint had been filed.

Gobie told the Times he always left Frank's official congressional plate on the dashboard whenever he used the car.

Frank has said he paid Gobie $80 for a sex act after answering a classified ad in a Washington gay newspaper in the spring of 1985.

The representative said he then hired Gobie as his housekeeper and driver, hoping to reform him, but fired him when he found out Gobie was using his apartment to operate a prostitution ring.

In today's statement, Frank said ''these and all other issues are rightfully before the House ethics committee at my request and will be dealt with in detail.''