Intravenous gammaglobulin (IVIgG) was given to seven adults with chronic idiopathic thrombocytopenic purpura (ITP). In parallel with platelet count, we studied Con A induced suppressor cell function for the autologous in vitro B cell response, T cell subsets and PAIgG levels, immediately before and 3–4 d after completion of treatment. Before treatment all patients had normal T cell subsets, decreased T suppressor cell function and increased levels of PAIgG. After IVIgG infusions, two patients were unresponsive and neither suppressor cell function nor PAIgG levels varied. In contrast, in the five responding patients we found an improvement in suppressor cell function and a decrease in PAIgG levels, while T cell subsets remained unmodified. Such results were reproducible in three patients after a second series of gammaglobulin infusions. Our results support the hypothesis that IVIgG infusion, apart from its effects on the reticuloendothelial system, may modulate the immune response by enhancing suppressor T cell function.