The frequencies of lymphocytotoxic antibodies i.e. anti HLA-ABC antibodies and anti HLA-DR antibodies, were studied in 66 women at delivery. They were, respectively, 18.2% and 9% in the first pregnancies, 27.3% and 4.5% in the second pregnancies, 50% and 27.3% in the multigravidous women (third pregnancy and more). The frequencies of anti HLA-DR antibodies were lower than those of the anti HLA-ABC antibodies and both seemed to increase with the number of pregnancies. The specificity of anti HLA-DR antibodies could be defined in only a few cases. The high frequencies of anti HLA-ABC antibodies compared with most reports in the literature can be explained by the higher sensitivity of the microlymphocytotoxicity technique used in this study. As patients with demonstrable anti-HLA immunization have shorter random graft survival than patients without demonstrable antibodies, this screening could have clinical application for an improved policy of selecting "acceptable mismatches" in recipients of kidney allograft transplants.