Arthritis could be passively transferred with a serum concentrate from collagen arthritic rats to nude rats and cyclosporin-treated, type II collagen-tolerant rats. These findings suggest that collagen arthritis could be inducible by humoral immunity alone in the absence of cellular immunity to type II collagen or functional T cells. In addition, passive arthritis induced by anticollagen antibody is a mild, transient disease from which the animals normally recover and the rats that have recovered from passive arthritis are resistant to develop a second phase of arthritis following a second administration of anticollagen antibody or the subsequent challenge with type II collagen. However, when a serum concentrate was transferred while cyclosporin was administered continuously, transferred arthritis in cyclosporin-treated, type II collagen-tolerant rats lasted as long as cyclosporin treatment and arthritis was significantly enhanced compared to those of naive recipients. Further, enhancement and prolongation of passively transferred arthritis in nude rats was observed. Furthermore, treatment with cyclophosphamide reversed acquired resistance to collagen arthritis subsequent to recovery from passive arthritis. These findings suggest that suppressor T cells might, at least in part, affect the clinical course of collagen arthritis and reverse acquired resistance to arthritis.