The effects of a therapeutic course of human interferon-beta (Hu IFN-beta) on platelet function were evaluated in 10 patients with neoplastic disorders. Each patient received, by intramuscular route, 6 x 10(6) IU of Hu IFN-beta every other day for 2 weeks. Platelet count, packed platelet volume, platelet size and in vitro aggregation tests were evaluated before IFN treatment and weekly during the treatment. An overall increase of in vitro platelet aggregation was present in all patients after the first week, and persisted after the second week of treatment. This effect was most pronounced when ADP and collagen were used as aggregation inducers. No significant differences were observed in IFN-gamma, IL-6, IL-1 beta and GM-CSF serum levels before and after IFN-beta treatment; nevertheless a parallel rise in IL-1 beta and/or GM-CSF and platelet count was noticed. The results obtained indicate that the effects of IFN-beta on platelet function are most likely due to a direct influence on bone marrow.