1. The titers of the isoagglutinins anti-A and anti-B in pregnant women at term do not differ significantly from the isoagglutinin titers in normal male adult blood donors. This indicates that pregnancy itself does not ordinarily stimulate a rise in isoagglutinin titer.
2. The frequency of incompatible blood groups in the infants of 31 mothers with toxemia of pregnancy was slightly higher (29 per cent) than that in a series of 166 infants of mothers without toxemia (22.3 per cent), but the difference was not statistically significant.
3. Of 37 normal mothers whose infants had blood groups incompatible with theirs, 9 were found to have a significant rise in isoagglutinin titer one week post partum. An appreciable rise in isoagglutinin titer occurred under similar circumstances in 6 of 9 mothers with toxemia. The series are too small to permit definite conclusions regarding the significance of this in relation to toxemia. However, there appears to be no doubt that under certain circumstances parturition can stimulate a rise in the maternal isoagglutinin titer.
4. No correlation was found between the occurrence of Rh incompatibility and toxemia of pregnancy.
5. The data obtained furnish additional evidence to support the theory of multiple allelic genes of heredity of the Rh blood types and also Race and Taylor’s theory of inheritance of the Hr factor.