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Transplacental hemorrhage is reportedly detectable by erythrocytes containing fetal hemoglobin-F in the maternal circulation. The procedure in the present study is based on the premise that fetal hemoglobin is not eluted from red cells in blood smears by an acid buffer, whereas adult hemoglobin is removed. Erythrocytes containing hemoglobin-F were observed in blood smears of both virgin and pregnant BALB/c mice, but the relative number of these cells in the maternal blood increased during pregnancy, reached a peak around the time of parturition, and decreased thereafter. In BALB/c females pregnant by DBA/2 males, the relatively large numbers of acid-resistant erythrocytes in maternal blood were related to the maternal unresponsiveness to DBA/2 allografts. The fetal derivation of at least a fraction of these cells was suggested by the apparent activity of fetal immunogens in the maternal circulation. The blood of peripartum BALB/c mice multiparous by DBA/2 males was injected into normal, nonimmune BALB/c mice. The blood recipients were later challenged with an allotransplantable DBA/2 tumor. A low degree of immunity was indicated by a decreased size of tumor implants. Attempts to demonstrate vertical transmission of immune suppression from mothers to progeny were negative. These findings indicate that transplacental hemorrhage is a potential source of antigens involved in the induction of specific maternal unresponsiveness.