The efficacy, toxicity, and mechanism of effect of intravenous Anti-D (Winrho) were studied in 43 Rh+ patients with immune thrombocytopenia purpura (ITP) who had not undergone splenectomy and in three already splenectomized patients. The mean platelet increase for the 43 nonsplenectomized patients was 95,000/microL (median 43,000/microL). Children had greater acute platelet responses than did adults. Human immunodeficiency virus status and duration of thrombocytopenia did not affect response. Maintenance treatment was given to patients as needed: the average interval between infusions was 24 days. The three splenectomized patients had no platelet response whatsoever. Toxicity was minimal; infusions were completed in less than 5 minutes. The generally accepted mechanism of effect of Anti-D has been Fc receptor blockade by substitution of antibody-coated red blood cells for antibody-coated platelets. Evidence is presented suggesting that the effect of IV Anti-D is not limited to Fc receptor blockade, including: (1) no correlation of parameters of hemolysis with platelet increase; (2) a 48- to 72-hour delay before platelet increase; (3) a tendency of the change in monocyte Fc receptor I expression to correlate with platelet increase; and (4) increased in vitro production of antibodies to sheep red blood cells following IV Anti-D infusion.