Detailed clinical and laboratory data have been presented on 21 infants with erythroblastosis due to ABO incompatibility. A careful survey in one newborn service over a six-month period showed that there were 11 cases of erythroblastosis due to ABO incompatibility as compared to 7 cases of erythroblastosis due to Rh incompatibility, suggesting that the disease occurs with greater frequency than has been hitherto reported. No one test is available which can in every case be absolutely diagnostic of the disease. The criteria useful in suggesting the diagnosis are listed. Practically, the presence of a major blood group incompatibility between infant and mother with a negative Coombs' test, clinical jaundice in the first 24 hours, and a serum bilirubin level of more than 10 mg./100 cc. at 24 hours or less is sufficient to make the diagnosis in the absence of manifest infection. The same criteria for treatment as applied to Rh erythroblastosis should be applied to erythroblastosis due to ABO incompatibility in the light of our present knowledge.