Washed red cells of 197 cord blood specimens were tested with rabbit antihuman IgG under low ionic hypertonic conditions in the presence of K-90 polyvinylpyrrolidone. The red cells of all 13 type A(1) and 8 type B babies of type O mothers were agglutinated. The red cells of one type A(2) baby of a type O mother were also agglutinated,but the cells of 6 other type A(2) babies of type O mothers were not. None of 153 red cells of ABO compatible babies were agglutinated and, of 16 ABO incompatible babies whose mothers were not type O, the red cells of only one were agglutinated. None of these babies had either a positive manual IgG antiglobulin test or clinical evidence of active hemolytic disease. The mean hemoglobin value of antiglobulin positive babies was significantly lower than that of antiglobulin negative babies, but this was associated with type A rather than type B babies.