Two cases of Auto-immune hemolytic anemia (AHA) in the horse are described. The pathogenesis of AHA in man is related to the findings in the horses. Besides from routine hematological and biochemical investigations specific data were obtained from the erythrocyte osmotic fragility test, the Coombs test, the serum haptoglobulin level and the cold agglutinin test. The first patient, a six month old Dutch standardbred colt, probably suffered from an acute attack of cold-induced hemoglobinuria with severe anemia and acronecrosis of the tops of both ears and of several parts of the skin that had been in close contact with the cold floor. The second patient, a nine years old Friesian mare, showed a type of AHA resembling the acute hemolyte type of cold agglutinin disease in man. This patient had a clear septicemic picture, extensive bacteriological examination, however, was negative.