Particulate and manganese mass emissions have been measured as a function of mileage for four Escort and four Explorer vehicles using both methylcyclopentadienyl manganese tricarbonyl (MMT) gasoline at 1/32 g Mn/gal and MMT-free gasoline. The vehicles were driven on public roads at an average speed of 54 mph to accumulate mileage, and the MMT was used in half of the fleet starting at 5,000 miles. This is an interim report describing the particulate and manganese emissions at 5,000, 20,000 and 55,000 miles. These vehicles are scheduled to continue mileage accumulation to 105,000 miles with emission testing at 85,000 and 105,000 miles. Of the total manganese used in the fuel, between 6% and 45% was emitted as airborne particulate matter, and the remainder was probably stored in the engine, catalyst and the exhaust system. The percentage of the manganese that was emitted increased with mileage accumulation. The EPA reports that manganese emissions of this magnitude may represent a health hazard. Also about 8% of the manganese used in the fuel was found in the engine oil. The manganese that is stored in the engine, catalyst and the exhaust system will be analyzed when the vehicles reach 105,000 miles. The total particulate emissions were between 0 and 5 mg/mi increasing with mileage accumulation and with MMT usage. The regulated emissions are approximately 0.3 g HC/mi, 3 g CO/mi and 0.4 g NOx/mi. The regulated and non-regulated gaseous emissions will be described in companion reports.