Serological and hematological investigations are reported for a woman who had multiple intrauterine deaths due to anti-M. The mother was group O Ns and the husband's cells were shown to be group O MSs. In the serological examination during the third pregnancy anti-M antibodies were identified in her serum. The antibodies comprised IgM saline agglutinin at a titer of 256 at 4 degrees C and IgG agglutinin reacted in an indirect antiglobulin technique at a titer of 4,096 at 37 degrees C. After the intrauterine death at 14 weeks gestation of her fifth pregnancy she underwent plasmapheresis 6 times on a biweekly program. Approximately 2 liters of plasma were exchanged for 1 liter of plasma protein fraction and 1 liter of fresh frozen plasma. In her sixth pregnancy, intensive plasmapheresis was started from 2 months gestation and a total of 56 liters plasma were exchanged. The anti-M titer fell to 256. She delivered a live girl by induction of labor at 35 weeks. The child was group O MNs. Her red cells had a positive direct antiglobulin reaction and her serum contained IgG anti-M antibody. Phototherapy was carried out and the child developed normally.