1. Fetal cells were demonstrated postpartum in approximately 20 per cent of Rh positive mothers with ABO incompatible infants and in only approximately 5 per cent of Rh negative mothers with both ABO and Rh incompatible infants in the absence of sensitization or presensitization.
2. When the prenatal findings were combined with those postpartum, the incidence was considerably greater in both groups (52 per cent and 32 per cent, respectively).
3. Two cases of massive transplacental hemorrhage in heterospecific pregnancies are reported with almost a normal survival of a few cells postpartum.
4. Observations on protective mechanisms bearing on isoimmunization in heterospecific pregnancies are made.