Methods are described for assaying, in vitro, the capacity of a subject's lymphocytes to manifest suppressor function. PBLs are exposed to an agent stimulating suppressor cell formation and/or activation. Other PBLs are treated to induce a cell proliferative response, thereby producing responder cells. The responder cells are subjected to an antigen in the presence of the suppressor cells previously produced and in the presence of various concentrations of an amplifier. The procedure provides a means of ascertaining suppressor capacity of the subject's immune system, i.e., the subject's potential to express suppressor function, thereby permitting assay of severity of autoimmune disease, titrating dosage of immunomodulatory agents, and determining whether modulator therapy is feasible.