The specificity and frequency of irregular erythrocyte alloantibodies in serum obtained from 85 pregnant women managed by a total of 480 intrauterine transfusions for treatment of fetal erythroblastosis was examined over a 4-year observation period. 138 alloantibodies reactive in the indirect antiglobulin test were detected. Their specificities were widespread. The frequency of non-anti-D alloantibodies primarily responsible for fetal immunohemolysis confirmed by elution from fetal red cells increased to 8% compared with studies performed in the 70s. 16 (19%) patients developed additional alloantibodies after onset of intrauterine transfusion therapy. Regarding the fact of the high incidence of secondarily induced alloantibodies, the high prevalence of antibody mixtures and the occurrence of rare alloantibodies against blood group antigens with weak immunogenic potency, we concluded that many of the patients were 'high responders'. Therefore the role of fetomaternal transplacental hemorrhage induced by invasive intrauterine examination methods and transfusions is discussed here. It obviously has to be considered as the main cause of the immunohematologic complications.