A Virginia House committee approved a mandatory seat belt bill today, rejecting an argument by a motorcylist from McLean that reduced fatalities from seat belt use would cut down on the number of human organs available for transplantation.

"That was the first new argument I've heard," chuckled Del. George W. Grayson (D-Williams- burg) after a hearing that had largely stressed the positive effects of seat belt use in automobile accidents and assertions that the measure would infringe on individual rights.

David Denholm, who said he represented a McLean motorcycle club, said the bill represented the same "wrong approach to government" that resulted in a law requiring helmets for motorcyclists. Denholm's remark that traffic accident victims "are a prime source of organs for transplants" drew loud groans from the audience.

"He's saying, 'Let's trade a whole person for part of a person,' " said Dr. Peter Sim, a Newport News physician who specializes in emergency care.

A similar seat belt measure passed the house last year but was defeated in the Senate Transportation Committee, where supporters are uncertain of its prospects this year. It passed the House Roads and Navigation Committee by a 13-to-7 vote today and the House is expected to take up the measure next week.

Gov. Gerald L. Baliles has urged the assembly to pass the law this year.

Supporters of the bill filled a huge committee room, passed out tiny packages of Lifesavers candy, accident statistics and buttons supporting the bill, which was introduced again this year by by Del. J. Samuel Glasscock (D-Suffolk). "I have been wearing my seat belt for 20 years and only needed it one time . . . [and] it was nice to have," Glasscock told the legislators.

Under his bill, Virginia motorists riding in front seats would be required to use seat belts or face a $25 fine. The measure would exempt persons with medical excuses and for-hire vehicles such as buses and taxicabs. Virginia already requires that passengers under the age of 4 be strapped in safety seats or harnesses.

Glasscock said his bill would save about 200 lives a year as well as prevent thousands of injuries. States such as New York and Illinois, among other states and foreign countries, have reported sharp drops in fatalities and injuries since enacting seat belt laws, he said.

One of the key supporters of the bill is the Virginia Automobile Dealers Association. It is trying to head off federal regulations that would require installation of automatic safety locks or air bags if states with two-thirds of the nation's population do not pass mandatory seat belt laws by 1990.

Glasscock said today that his bill would not meet federal standards primarily because it includes a provision that says failure to wear seat belts cannot be used against someone in a damage liability case. He also said the exemption for taxicabs, approved by the committee today, would not meet the federal test.

Both Glasscock and Del. Robert T. Andrews (R-Fairfax) said they expect the federal standards to be eased soon.