Mechanisms of aggregation and release reactions of human platelets induced by non-hemolytic influenza virus were studied. The influenza virus (PR/8 strain) caused aggregation of platelets with ATP release in a dose-dependent manner over the range of 640-10,240 HA titers. The aggregation was always preceded by a lag period and subsequent shape changes. The virus-induced aggregation was enhanced by exposure of the reaction mixtures to cold at 4 degrees C for 30 min and inhibited by apyrase, acetylsalicylic acid and dipyridamole. Ingestion of acetylsalicylic acid also inhibited the aggregation. Gel-filtered platelets were aggregated by influenza virus only after the reaction mixtures had been previously incubated at 4 degrees C for 30 min and then stirred at 37 degrees C. In the absence of divalent cations (Ca2+ less than or equal to 2 x 10(-5) M, Mg2+ less than or equal to 1 x 10(-5) M), gel-filtered platelets were not aggregated by influenza virus. These results suggest that influenza virus was absorbed onto the platelet surface and caused the release of ADP from platelets, which in turn, aggregated platelets.