Interferon-α(IFNα) has been shown to increase platelet numbers in patients with immune thrombocytopenic purpura (ITP), but the basis for this effect is not known. In this study changes in immune function were monitored following administration of IFNa to seven patients whose ITP had proved refractory to conventional therapy. Patients' peripheral blood mononuclear cells were cultured with phytohaemagglutinin and culture supernatants assayed for cytokine production. Production of the Thl cytokines IL-2 and IFN-γ/γwas low in patients compared to healthy controls, and amounts of these tended to increase after IFNβtherapy. In two patients high levels of IL-10 were detectable in culture, and these were reduced after IFNa. In patients who had high serum concentrations of IL-4 or IL-10, these were also decreased following therapy. Natural killer cell activity, which was low in the patients prior to therapy, was increased following administration of IFNβ. In those patients with detectable platelet-associated autoantibody to gpIIb/IIIa, the amounts were reduced after treatment. Two patients had an unusually high percentage of T cells expressing theγ/γ§T cell receptor, which decreased after therapy, The findings are consistent with an increase in Thl activity and a decrease in autoantibody production following IFNβtherapy, which may be related to the beneficial effects of this cytokine.