In addition to a lupus-like syndrome and massive T cell proliferation, MRL-1pr/1pr (MRL/1) mice develop an arthritic process very similar serologically and histologically to human rheumatoid arthritis (RA). Recently, we have developed in DBA/1 mice an experimental model of autoimmune arthritis (EAA) which shares clinical features with RA, by injecting homologous type II collagen (CII). In order to investigate the possible relationship between the spontaneous polyarthritis of MRL/1 mice and collagen induced EAA, we immunized MRL/1 mice with mouse (M) CII. Our findings revealed that the injection of 100 /jg M-CII in young or old MRL/1 mice did not modify the articular pathology which spontaneously develops in non-injected mice. Circulating autoantibodies to native M-CII were found in the sera of immunized young mice but were not detected in non injected or immunized old mice. Conversely, denatured alpha 1 (II) chains or CB peptides derived from M-CII were recognized by most of the MRL/I sera whether mice had been immunized or not. The incidence of positive sera as well as the intensity of the response evaluated by Western blot analysis increased with the age of the mice. Taken together, our data suggest that, even if the injection of homologous CII in MRL/1 mice may accelerate the onset of joint pathology, the spontaneous disease arises independently of an autoimmune response against native CII