Experimental immunotherapy of murine collagen-induced arthritis can be achieved by administration of specific T suppressor cell hybridomas. The present study examines the immunoregulation noted in this experimental immunotherapy by describing the immunomodulatory effects of a cytokine produced by a suppressor T cell line, T101N. The inhibition of the activation of splenic lymphocytes in response to T cell mitogens and the erythema and edema associated with arthritis were assayed. Mice given T101N ascites showed reduced inflammation (p<0.05). Lymphocytes derived from naive mice and cultured in the presence of T101N culture supernatant showed reduced response to concanavalin A. Therefore, this form of experimental immunotherapy of arthritis may be associated with cytokines secreted from T suppressor cells which modulate T cell activation.