Particulate emissions from 20 light-duty gasoline passenger cars and trucks were characterized using the Federal Test Procedure and Highway Fuel Economy Test driving cycles. Emission patterns were examined with 4 of the vehicles using three additional driving cycles, the hot start Federal Test Procedure, the Congested Urban Expressway and the New York City. The test fleet consisted of 4 noncatalyst vehicles operated with leaded gasoline and 16 catalyst-equipped vehicles operated with unleaded gasoline. The vehicles, obtained from local rental agencies, dealers, and residents, ranged in age from model year 1970 through 1981 and in mileage accumulation from about 300 to 81,000 miles.Particulate characterization included determination of total particulate emission rates, Ames bioassay of the dichloromethane soluble organic fraction, and analysis of the nitropyrene, pyrene, benzo-a-pyrene, and trace elements content. Hydrocarbon, carbon monoxide, and nitrogen oxides emission rates, as well as fuel economy were also determined. The gasoline vehicle results were contrasted with the results from a similar diesel program previously reported. Generally, hydrocarbon, carbon monoxide, and nitrogen oxides emission rates were lower from the in-use diesels than the in-use gasoline vehicles tested, and total particulate emission rates higher from the in-use diesels. The mutagenic activities, as indicated by Ames Salmonella strain TA-98 bioassay, of the particulate organics emitted from the gasoline vehicles were higher than from the diesels. However, the higher activities were compensated by lower particulate organic emission rates, especially so for catalyst-equipped vehicles.