In vitro immune function was assessed in patients with multiple sclerosis (MS) who were receiving Imuran therapy, in untreated MS patients, and in controls. In untreated stable MS patients, concanavalin A (Con A)-driven mitogenic reactivity (T effector function) and Con A-induced suppressor activity were modestly reduced compared to controls; pokeweed mitogen-induced immunoglobulin G (IgG) secretion was increased. Untreated patients with active MS demonstrated high levels of IgG secretion and marked decreases in suppressor activity. In Imuran-treated patients, Con A mitogenic responses and suppressor activity were comparable to those observed in untreated stable patients, and IgG secretion was reduced. The results in the treated patients likely reflect a direct effect of Imuran on B cell function rather than an indirect effect mediated via suppressor cells.