In Hungary, road vehicles are responsible for a significant part of the air polluting emissions in the great cities especially in the capital - Budapest. To mitigate this problem one of the most promising solutions is the fitting the in use cars with catalysts (so called retrofitting). In this way the technical potential is given, on the one hand, by the large availability of unleaded petrol in Hungary and on the other hand, by the large share of the home car fleet (about 80%) which can be fueled with unleaded petrol.The field test aimed at the evaluation of large scale use of retrofit catalysts (in real life), especially in the case of two-stroke engined cars. During the tests, first of all the useful life-time expectation and the alteration of the reducing efficiency of CO, HC and NOx were investigated. Beside these, the light-off temperatures and thermal load of the catalysts and their impact on the fuel consumption and dynamic features of the cars were also examined. The test catalysts were purchased from seven well known European manufacturers and equipped on 70 cars typical of the home car fleet. The efficiency of the emission reduction was evaluated by the European driving cycle tests carried out before and after the installation of the catalysts as well as at several mileages.The results justified the governmental support of retrofit catalysts and their large scale application. The average efficiency regarding CO, HC and NOx was in all cases over 50%, and no definitive deterioration had been experienced up to 50-60.000 km, in the case of four stroke engined cars and up to 20-30.000 km, in the case of two stroke engined ones. The NOx efficiency suggests that the use of TWC without closed loop control should be preferred over oxidation catalysts. Only technically qualified catalysts having high technological level should be recommended for retrofit installation. A nationally accepted test method was determined for this purpose.