Sera from 98 primiparous Rh(D)-negative women were tested for leucocyte isoimmunization from blood samples taken at delivery and 3 months after, when prevention of Rh immunization was first checked. Lymphocytotoxic antibodies were demonstrated already at delivery in 16 cases (16.3%). Ten of the remaining 82 women whose serum was negative at delivery developed antibodies in the meantime, according to the tests carried out with serum samples taken 3 months after (12.2%). All batches of anti-D IgG used during the investigation period contained lymphocytotoxic antibodies. However, a protective effect against leucocyte isoimmunization is unlikely in these cases. More than half of the immunized women were already primed by the time of delivery, and a considerable percentage (12.2) of the rest became immunized afterwards. The incidence of immunization detected either at delivery or 3 months after,altogether 26.5%, moderately exceeds the figure for the indidence of lymphocytotoxic antibodies during second pregnancy in the authors’ own statistics, obtained with the same serological method (21.6%). In 46 cases at and after second delivery a similar relation was found with antibody incidence during third pregnancy.