Allogenic leukocyte immunization is one of several treatments tried for unexplained recurrent aborters, and is reported to have few maternal and neonatal side effects after the immunotherapy having been reported to date. In the present study, we report a rare case of neonatal thrombocytopenia (41000 cells/microl) observed in a female infant delivered by an unexplained habitual aborter. The mother was immunized with her husband's leukocytes once before pregnancy and twice at the 5th and 6th week of her successful pregnancy. Serological studies using mixed passive hemagglutination assays (MPHA) showed that maternal serum did not contain any antibodies which were reactive to 11 platelet-specific antigens, or to granulocyte antigens extracted from 9 persons. Lymphocyte cytotoxicity tests, however, showed that maternal serum but not infant serum had anti-HLA antibodies against both paternal and infant lymphocytes. Moreover, the maternal serum was found to have anti-HLA IgGs against platelet antigens extracted from the father and the infant. It is highly likely that this case of neonatal thrombocytopenia was caused by transplacental perfusion of maternal anti-HLA antibodies whose production was induced or enhanced by the allogenic leukocytes immunizations.