Type II native bovine collagen was used to induce arthritis in sheep. The sheep received a subcutaneous injection of collagen in Freund's adjuvant, and some sheep also received an intra-articular injection of collagen. The response to the collagen injections was monitored by measurement of antibody levels to bovine and ovine collagen in serum, synovial fluid and afferent lymph samples. There were antibodies in the normal sheep serum which reacted with native and denatured type II collagen, but there was a marked rise in serum and synovial fluid reactivity after subcutaneous injection of native collagen. In these sensitised sheep there was a further rise in antibody levels in the synovial fluid and afferent lymph samples after an intra-articular injection of collagen. There was little effect on antibody levels in the non-injected contralateral limb. Histological and immunohistochemical studies showed that the rise in antibody levels was associated with pathological changes to the synovium which were consistent with the development of arthritis. These changes were most severe in those joints of sensitized sheep which had received an intra-articular injection of collagen and were characterized by increased numbers of T cells in the synovial tissue, mainly of the CD4 phenotype. There was a marked increase in B cell expression by cells on the surface of the synovial intima, and accumulations of B and T cells were seen in the subsynovial tissues. Collagen-immune complex arthritis in sheep appears to be a useful large animal model of arthritis, with significant similarities to the arthropathies of other species, including man.(ABSTRACT TRUNCATED AT 250 WORDS)