When rats were injected intradermally with an oil emulsion of native type II collagen, they developed an inflammatory polyarthritis. The incidence and severity of arthritis increased as the amount of collagen injected was increased. Rats 4 1/2 weeks old were the most susceptible to the development of arthritis, whereas weanling and older animals were relatively resistant. There was no difference in incidence between males and females. Mononuclear cells from peripheral blood, lymph nodes, and spleen were cultured with type II collagen and responded maximally to a collagen concentration of 25 microgram/ml. The earliest detectable response was in peripheral blood mononuclear cell cultures obtained 6 to 8 days after immunization. The response of lymph node and spleen cells tended to lag behind that of peripheral blood cells at the earlier time intervals. Antibodies were detected in sera by hemagglutination at 8 days postimmunization. Quantitation of IgM and IgG antibodies by radioimmunoassay showed good correlation with hemagglutination titers and increased binding of collagen by both classes of antibody in arthritic as compared to nonarthritic animals. It is clear that the development of both humoral and cellular immunity to type II collagen is associated with the development of arthritis and may be important in the pathogenesis of this disease.