A black Cadillac limousine with American flags flying from the front fenders rolled up to the Senate Office Building today and unloaded a half-dozen shaggy motorcyclists who come to lobby in behalf of legislation that will permit them to ride their bikes without wearing helmets.

The Cadillac, explained driver Paul Jackson, 24, of Hillcrest Heights, is "the official car of Prince George's ABATE (A Brotherhood Against Totalitarian Enactments)" and its occupants had come to the state capital to restore their constitutional right to "get our heads bashed in if we want to risk it," one of them explained.

The freespirited cyclists crammed the hearing room of the Senate Committee on Constitutional and Public Law in support of a proposal by Sen. Peter A. Bozick (D-Prince George's) that would demand the existing requirement for helmets so that it would apply only to riders under age 18.

Alfred Sampedro, a 26-year-old ironworker from Suitland, complained that the law "infringes on my personal rights." He doesn't mind wearing it for dirt racing or scrambling, but when he's on the open road, he wants it off.

It was an unlikely group of civil liberatarians that packed the hearing. Many wore their scars like battle decorations on their uniforms of leather jackets, sequined jeans, mirrored sunglasses and golden rings in their left ears.

Gary R. Zager, 30, refuses to wear a helmet, despite the law (he was arrested 11 times last year) and says that decision has nothing to do with disfigurement of his face.

"A helmet won't save your face," said Zager, who has been knocked off his bike twice in the last three years by cars that pulled into his path. Zager, a telephone repairman from Clinton, is Maryland director of ABATE, a nationwide organization that seeks repeal of helmet laws, an effort that has been successful in a dozen states.

Bill Foster, 25, suffered a skull fracture when his bike was run over by a three-quarter-ton truck five years ago, but Foster believes that if he had been wearing a helmet "it probably would have killed me."

Foster works in the emergency room of the hospital in La Plata and has seen many of his buddies hauled in on bloody stretchers, but observed that "most of the bad ones had been in cars."

James Adkins, 25, of Forestville, has been in three crashes, has two metals pins in his shoulders and one in his left knee, but said that his head did not touch the ground in any of the smashups. "The damage is unpredictable," Adkins said.

William Cranston, 24, a plumber from Hyattsville got rear-ended by a dump truck last Oct. 21 on Rte. 202 and suffered knee and neck injuries.He blames the knee on the truck, the neck on his helmet.

While most of the cyclists who attended the hearing were of the beard-and -leather jacket set, those who spoke in their behalf were not.

Francis Patrick McCamley, vice president of the National Capital Motorcycle Club, is 41, the father of six and officer of a labor union. He said helmets limit a rider's sight and hearing and give "a false sense of security."

Ed Armstrong, of Fox River, Ill., a member of Illinois ABATE and a registered engineer, said that available helmets are not strong enough to prevent injury unless the impact is less than 4 miles per hour. An effective helmet would not be wearable, he said, because it would weigh 15 pounds.

Major opponent of relaxing the law are the State Police and the state Motor Vehicle Administration. Members of the Senate committee were given copies of a report by the Motor Vehicle Administration's office of highway safety programs that recommended delay of any repeal action "because of the demonstrated effectiveness of helmet use in reduction of serious and fatal head injuries."

That finding also was supported by testimony from Dr. J. Crawford McAslan, associate clinical director of the University of Maryland Hospital's shock trauma unit. He said the unit's team of physicians has been able to "significantly reduce the death rate" of accident victims except those with head injuries, and that one-sixth of all those admitted with head injuries were motorcyclists.

State Police Lt. N. H. Tooren said that in states where helmet laws have been repealed, "all police officers use them anyway" and none approves of the repeal.

After the hearing, ABATE members challenged Tooren's statement. They said a survey among Massachusetts members of the Blue Knights Motorcycle Club, composed entirely of law enforcement officers, found its members favored repeal. A large number of the officers said they would continue to wear helmets, but had concluded that "you cannot legislate common sense."

As he kicked aside a carton of empty Miller High Life bottles on the floor of the ABATE limousine, Cranston, the plumber, said repeal would make him happy. "You can tell a happy rider by the number of "bugs" on his teeth when he smiles," advised Cranston.