Two alloimmunized patients with multispecific anti-HLA and high-titered ABH antibodies showed transfusion failures after ABH-mismatched HLA-identical platelet transfusions, whereas ABH-matched HLA-identical platelets showed sufficient increments. The anti-A and -B could be demonstrated on platelets by immunofluorescence tests using FITC-labeled goat anti-human IgG. These platelet antibodies could be absorbed with red cells and platelets of the appropriate ABH type. In contrast to previous reports about the influence of ABH antibodies on platelet survival, not only a proportion of the platelet population were destroyed after infusion, but almost all ABH-incompatible platelets were destroyed.