Native type II collagens of rat, chick, bovine, human and murine origin have been used for immunization of male and female DBA/1 mice of different ages. The four heterologous collagens were able to induce arthritis in both male and female mice although the incidence of arthritis was higher in males. The onset of arthritis was sudden with severe lesions starting to appear 5- to 6-weeks after immunization. Mouse collagen II, on the other hand, caused arthritis exclusively in males. The onset of arthritis was in the latter case less dramatic and usually delayed until 12- to 14-weeks post immunization. Antibody production against both the immunogen and other type II collagens, including mouse collagen was seen in both arthritic and non-arthritic animals, and antibody titres against both the collagen used for immunization and mouse collagen were higher in the sera from arthritic than in identically immunized non-arthritic animals. Total amounts of auto-anticollagen II antibodies were, however, more dependent on which collagen preparation was used for immunization than on whether the immunized mice developed arthritis or not. These results indicate that arthritis induction in mice is not dependent solely on the levels of autoreactive anti-collagen II antibodies.