Immunohematologic and clinical data, i.e., antibody profile, location of the placenta, mode of cordocentesis, obtained from 48 pregnant patients with irregular erythrocyte antibodies during the last 2 years have been retrospectively evaluated. All fetuses of the patients received intrauterine transfusions for the treatment of fetal erythroblastosis. In 16 (33%) patients (group I) a secondarily induced antibody was detected after the onset of intrauterine transfusion therapy. 32 (67%) patients (group II) did not further develop new antibody specificities. Group I exhibited a significantly different distribution in the location of the placenta (p < 0.05; chi 2 test) as compared with a nonselected control group of pregnant women. In group I a 5-fold higher rate of anterior than posterior placenta location was found. The mode of cordocentesis differed significantly (p < 0.01; chi 2 test) between group I and group II patients. In group I a significantly higher rate of transplacental punctures than in group II had been performed. This has to be considered to be causative for the secondary sensitization. Therefore, the secondary induction of antibodies by invasive intrauterine interventions in our patients depended indirectly on the location of the placenta and directly on the mode of the puncture (trans- vs. paraplacental access).