A family is reported with multiple cases of neonatal neutropenia. The first child died of severe infection, the second was asymptomatic, the third and the fourth children had a benign course of agranulocytosis which lasted about 10 weeks and was followed by spontaneous recovery. A potent leukoagglutinin was found in the maternal serum which agglutinated leukocytes obtained from the father and all three available children but failed to agglutinate the mother’s own cells. Identical leukoagglutinin was found in the last baby’s serum; it disappeared after the baby recovered. The disorder is tentatively named "isoimmune neonatal neutropenia" and is considered to represent a condition analogous to erythroblastosis fetalis.