In the course of antigen-induced arthritis of rabbit cell-mediated and humoral immune responses were repeatedly tested in order to prove their significance for the acute and chronic phase of inflammation. The arthritis was monitored during the progression of the inflammation by means of the joint swelling and at the end of experiments by histological evaluation of synovitis and cartilage degradation. Following the arthritis induction a strong increase of specific antibodies and of circulating immune complexes was evident. The correlations between antibody levels and joint swellings confirmed that the local formation of immune complexes is responsible for the initiation and perpetuation of arthritis. In the early phase after immunization the responsiveness of lymphocytes to antigenic and mitogenic stimulation was increased, in the late chronic phase of arthritis proliferative responses of lymphocytes to cartilage matrix components were revealed. No direct correlations could be demonstrated between any cell-mediated immune response and the severity of arthritis. The hyperreactivity of cell-mediated immunity is suggested to be responsible for the transition of the acute arthritis into the chronic stage. The deficiency of an effective suppression results in the activation of B-lymphocytes with increased production of antibodies, maintaining the inflammatory process for a long time. Under these conditions the release of cartilage matrix components during the acute joint reaction induces autoimmune responses against cartilage, which could contribute to the chronification of arthritis and to cartilage degradation.