Several animal models of arthritis are produced using complete Freund's adjuvant (CFA) alone or with collagen as an arthritogen. Successful induction of arthritis is reported to require that the adjuvant mixture be administered by intradermal or subcutaneous routes. The resulting arthritis is caused by primarily cellular immune responses. Data presented in this paper show that giving CFA by intraperitoneal (I.P.) inoculation results in a humoral autoimmune response, with no obvious signs of arthritis. This humoral autoimmune response is characterized by production of autoantibodies to nuclear and cytoplasmic antigens, elevated levels of circulating immune complexes, and in approximately 25% of mice, rheumatoid factor.