Isoimmunisation to platelet and lymphocyte antigens occurred during a woman's seven pregnancies and resulted in congenital thrombocytopenia and lymphocytopenia in the last two offspring. The youngest baby died aged 16 days of severe combined immunodeficiency (SCID) with maternal graft-versus-host disease. The other affected child, however, recovered fully from thrombocytopenia and lymphocytopenia after exchange transfusion for presumed sepsis. The clinical courses indicated that an isoimmune process had caused both the thrombocytopenia and the lymphocytopenia. In accord with this conclusion, maternal titres of IgG antibodies against paternal platelets fell continuously in serial blood samples taken after the last pregnancy. Cytotoxicity studies of the maternal serum towards lymphocytes from the other siblings and from unrelated donors showed that the paternal antigen involved was a non-HLA determinant. It is postulated that isoimmune lymphocytopenia with subsequent immunological suppression is a potential mechanism for SCID and intrauterine engraftment of maternal lymphocytes.