Concanavalin-A-induced T-suppressor cell activity was decreased in spleen cell suspensions from Lewis rats with adjuvant arthritis. This decreased activity was evident on day 10 after the induction of the disease, just before the development of the polyarthritic lesions, and persisted during the period of active inflammatory and immunological disease. The extent of the impairment of T-suppressor cell activity was positively correlated with the severity of the arthritic lesions. Removal of phagocytic cells prior to induction of T-suppressor cells abolished the observed decrease in suppressor cell activity. It is suggested that this model may be of value for the investigation of suppressor cell function in immunologically mediated disorders.