We studied the mechanism of platelet injury in 20 patients receiving heparin, three of whom became thrombocytopenic. Platelets from these three patients had increased levels of IgG and C3, which correlated with the presence of thrombocytopenia; their plasma caused the release of serotonin from normal platelets at concentrations of heparin within the usual therapeutic range. This reaction required IgG and an intact classic complement pathway. The 17 patients receiving heparin in whom thrombocytopenia did not develop had normal levels of platelet-associated IgG and C3. Plasma from 14 of these patients caused the release of serotonin, but only at heparin concentrations above the therapeutic range, in a reaction that also required IgG and complement. The addition of heparin to 15 normal plasma samples did not cause platelet injury in vitro. These studies indicate that heparin administration can be associated with complement-mediated platlet injury. The dose-dependent nature of this process may account for the occurrence of thrombocytopenia in some of these patients.