Prince George's County police said yesterday are taking license numbers of cars parked near massage parlors, tracing them and inviting their owners to police headquarters for an interview.

The move is designed to crack down on what police describe as suspected illicit activities, such as prostitution. Undercover surveillance will be undertaken at some, if not all, of the country's 16 massage parlors -- police wouldn't say which ones. Persons whose cars are observed nearby can expect a letter.

The letter says that if the car's owner fails to come in for an interview to discuss and "unusual activities" in a massage parlor, he or she will be recontacted by police in five days. The letter doesn't spell out whether the "recontact" would be by phone or in person or whether it sould be attempted at the home of the car's owner.

"The letter may be perceived by some as a form of harassment of dirty tactics," the police department acknowledged in a statement, "but [the department] feels that the purpose of the investigation is to bring to a close any illicit activities that may be going on under the guise of a massage parlor."

County Police Chief John E. McHale said he wants to see the elimination of every massage parlor in the county that isn't "legitmately helping people with back problems."

The surveillance program is the lattest effort of the Prince George's government to crack down on the parlors and "health spas" that have sparked dozens of complaints from their neighbors. "No question, the parlors have given the county a bad image," McHale said.

Two months ago County Executive Lawrence J. Hogan asked the County Council to pass emergency legislation tightening county law governing massage parlors and health clubs, particularly when they are believed to be prostitution fronts. That bill is pending before the council.

McHale said he believes that the legislation, if enacted, would be the most effective method for combating illegal activity in the parlors. But county police said they hope the publicity surrounding the new surveillance program will immediately "deter a portion of the prospective partrons."

A draft of the letter police plan to send to persons whose cars are seen near massage parlors, made available to The Washington Post, says:

"On [date] at --- hours your vehicle, tag number -----, was observed by a surveillance team at -------. We realize that you may have loaned your vehicle to someone else or it may have been left at this location for some legitimate purpose. Because of the sensitive and confidential nature of the investigation I ask that you contact [the police] so that they can interview you as to any criminal or unusual activities you have observed if in the establishment."

The letter then says its recipient has five days from the date of the postmark to contact the police, or the police will "recontact you."