A model of arthritis was established by the injection of type II collagen into mice. Only mice bearing the H-2q haplotype were susceptible to the disease. Susceptibility was further mapped by the use of recombinant strains on the Iq locus. Type II collagen arthritis was observed in the (resistant X susceptible) F1 cross. Mice strains were designated high, intermediate, or low responders with respect to the anti-type II antibody levels measured by radioimmunoassay. Arthritis-susceptible strains were all classified as high antibody responders. The clinical and histological appearance of type II collagen arthritis in the mouse indicates that it may be a good animal model for the investigation of various immunogenetic traits in rheumatoid arthritis.