Severe degrees of intravascular red cell agglutination have been observed during life in rats injected with anti-rat red cell serum and in a patient of 11 months with chronic acquired hemolytic anemia. There was no significant fall in the tissue oxygen tension or in the total oxygen consumption of the rats. Neither the rats nor the infant developed kernicterus. In a baby dying with kernicterus no true hemagglutination was observed, although there was slight sludging such as is seen in many illnesses. Additional support for the belief that the red cells in this case were not clumped lies in the fact that in vitro clumping was not observed; furthermore, the sedimentation rate was only 1 mm. in one hour. The blood of the injected rats and of the other infant who did not develop kernicterus sedimented extremely rapidly and displayed spontaneous agglutination in vitro. These observations indicate that intravascular agglutination has little if any bearing on the development of kernicterus in erythroblastosis fetalis.