Gold complexes are used in the treatment of rheumatoid arthritis for some 60 years by now. The authors used therefore a gold complex, sodium aurothiosulphate (ATSS) to influence the experimental model of arthritis induced by immunization with type II collagen in laboratory rats. To the first group of laboratory rats ATSS was administered concurrently with the first immunization dose, to the second group with the second immunization dose and to the third group in the course of arthritis. ATSS was administered to individual groups every week by the i. m. route, 20 mg/kg body weight. In all three groups a reduction of arthritic symptoms was observed, however, in group three to a much lesser extent than in groups one and two. The results of the experiment indicate clearly that ATSS was able to suppress the development of collagen induced arthritis, if administered not later than with the second immunization dose. As the formation of antibodies against type II collagen was not suppressed, it may be assumed that the activation of the complement system was blocked by the bond of the gold complex with the C1q component. It has been proved already previously that the interaction of C1q with gold complexes is very rapid.