An acute inflammatory arthritis has been induced in 76% of rats injected intradermally with native bovine type II collagen emulsified in Freund's complete (CFA) or incomplete (ICFA) adjuvant. The arthritis became chronic in 14 out of 31 rats, and ear and tail lesions were noted in some rats. No arthritis was induced by native type I collagen, denatured type II collagen, rabbit IgG, or buffer alone injected intradermally with adjuvant. Using a solid-phase radioimmunoassay for serum antibodies we have shown that IgM and IgG levels to native bovine type II collagen were significantly higher in arthritic than nonarthritic rats. Antiglobulin antibody levels were not raised in arthritic rats. Equivalent antibody levels to native type II collgen were obtained whether this antigen was emulsified in ICFA or CFA. However, native type I collagen produced high antibody levels only when emulsified in CFA, indicating a difference in the immunogenicity of these collagens. These studies suggest that native type II collagen possesses arthritogenic properties in the rat and that humoral immunity may play a role in the induction of this arthritis.