mAb were developed from DBA/1 mice immunized with chick type II collagen. A total of 69 IgG antibodies was isolated and characterized. The majority (36%) reacted with a CNBr-derived peptide CB11 previously identified as containing a major immunogenic and arthritogenic epitope(s). Seven of the antibodies reactive with CB11 crossreacted strongly with mouse type II collagen. These were administered to DBA/1 mice in an attempt to induce arthritis. Individual antibodies were able to induce mild lesions consisting of minimal synovial proliferation but not overt arthritis. However, a combination of antibodies induced severe arthritis with marked destruction of articular cartilage. The minimal effective combination consisted of three antibodies. Arthritis developed within 48 to 72 h after injection of the antibodies and persisted for the duration of the observation period of 3 wk. Antibody levels were measured at intervals and persisted for the 3 wk observation period although at diminishing levels. Competitive binding assays demonstrated that each of the effective antibodies bound independently suggesting that some spatial or quantitative relationship was important possibly related to their ability to activate complement.