Frank Aluisi, chairman of the board that operates three Prince George's County health facilities, is under investigation by the board's ethics committee because he took a friend on a trip to a Mexican medical school last year. The airfare was financed through a private educational fund that is supported through contributions from doctors.

Last December, Aluisi and Catherine Kilbridge accompanied two doctors from Prince George's General Hospital to the school in Guadalajara to interview residency candidates.

Aluisi is chairman of the board of Community Hospital and Health Care Systems Inc., a private corporation created three years ago by the County Council to run Prince George's General and Greater Laurel-Beltsville hospitals and the Bowie Health Center. The organization has been criticized recently by county officials who say the facilities are poorly run.

County attorney Tom South said that some county officials were upset when they learned yesterday about the trip and that he has contacted the board to let members know that "we would encourage the corporation to take prompt, public action . . . . I think resignation is a viable alternative."

The president of the hospital corporation had no comment.

Aluisi said yesterday that he was invited to go on the trip by members of the hospital medical staff. He said he took Kilbridge but he had intended to pay for the trip. He said the medical staff never forwarded a bill to his office.

Aluisi said he paid the hotel bills himself and estimated that the airfare for himself and Kilbridge was about $1,000. He said that yesterday he instructed a secretary to draw the money for the bill out of the funds that he is paid for his duties as board chairman.

Kilbridge said last night that she was Aluisi's guest and assumed he had paid for the trip.