Five independent alloantibodies directed to factor VIII coagulant antigen (VIII:CAg) were assessed against normal, von Willebrand's disease, and severe hemophilia A plasmas. Immunoradiometric assays (IRMAs) were developed for each antibody, one of which had arisen “spontaneously” and four in transfused hemophiliacs. The correlations between assays were very high for normal, vWd, and both CRM+ and CRM- hemophiliacs. This suggests that IRMAs maybe developed from almost any reasonably high titered alloantibody and used with confidence in diagnosing CRM- hemophilia A in utero by fetoscopy.