Monoclonal antibodies to certain cell surface constituents on lymphocytes, monocytes and macrophages have been administered to Lewis rats with developing, established or adoptively transferred arthritis, to determine any immunomodulatory properties. Anti-CD4 antibodies against helper T-lymphocytes produced a dose related inhibition of developing arthritis; high dose levels completely suppressed all symptoms of arthritis and these rats were resistant to further attempts to induce arthritis. Anti-Ia (MHCII) antibodies also inhibited arthritis in a dose related manner; anti-pan T antibodies delayed the onset of arthritis, but antibodies against CD8 and IL-2 receptor positive cells were without effect. Development of type II collagen-induced arthritis was also inhibited by anti-CD4 treatment. Established arthritis could be temporarily inhibited by anti-CD4 antibodies, but rebound of arthritis invariably occurred after stopping treatment, as is the case with cyclosporin A. Similar results with anti-CD4 antibodies were obtained during treatment of arthritis adoptively transferred by arthritogenic T-lymphocytes. From these experiments it is clear that CD4 positive T-lymphocytes have a major role in the induction of adjuvant arthritis and that interaction between CD4 and Ia bearing cells is important. The rebound of arthritis that occurred after withdrawal of anti-CD4 treatment during established disease infers that cells in addition to helper T-lymphocytes are involved in the chronicity of arthritis, but these remain to be elucidated. These findings are discussed in relation to results with monoclonal antibodies in other models of arthritis and human rheumatoid arthritis; the prospects for human therapy are also discussed.