A fleet of nearly 250 cars equipped with experimental catalytic converter systems were tested in taxi, police, state, and municipal fleets in various cities throughout the country. This provided a diversified range of customer service and altitude and climatic conditions. The objective was to evaluate the performance and durability in high mileage field service of experimental catalytic emission control systems. The fleet comprised groups of cars with hardware and calibration variations designed toward the 1975 Federal and California and more advanced emission requirements. The converter systems evaluated were primarily a 260 cubic inch underfloor converter and a 140 cubic inch manifold converter. Both bead and monolith substrate catalysts were examined.Test results showed that on the average the systems successfully controlled emissions to below the 1975 Federal and California requirements for greater than 50,000 miles. Engine misfire conditions did cause converter damage in some instances. Systems designed for the low emission requirements of .41/3.4/.40 grams/mile HC/CO/NOx exceeded those levels at relatively low mileage due to catalyst deterioration.