In vitro phagocytosis of platelets from patients with idiopathic thrombocytopenic purpura (ITP) (direct assay) and phagocytosis of normal platelets sensitized with patient serum (indirect assay), using normal peripheral blood leukocytes, were studied to find out whether the degree of phagocytosis reflects the severity of ITP. Phagocytosis was measured by the uptake of 51Cr-labeled platelets. Among patients whose platelet count was below 10 X 10(4)/microliters, 92% showed an increase in the phagocytosis ratio by the direct assay and 67% by the indirect assay. There was a highly significant inverse correlation between the direct phagocytosis ratio and the platelet count (p less than 0.001), but not between the indirect phagocytosis ratio and the platelet count. The amount of platelet-associated IgG (PAIgG) measured in patients whose platelets were used for the direct assay also showed an inverse correlation with the platelet count (p less than 0.05). Vincristine administration provoked a reduction in the direct phagocytosis ratio and PAIgG value. We propose that both the direct assay and PAIgG measurement reflect the severity of ITP, and PAIgG might be responsible for platelet phagocytosis.