Four litters of newborn piglets suffering from haemolytic disease of four different grades of severity, were observed clinically. Pallor and mild jaundice were evident in some cases, but the activity of the piglets was not reduced except in the terminal stages of fatal cases. Malnutrition affected a number of piglets sometimes from fortuitous factors. In mild cases there were no clinical symptoms, and the disease could be detected only by demonstrating a fall in haemoglobin level or sensitization of the red cells.

Several blood group systems were concerned in the genesis of the disease in these piglets and both agglutinating and incomplete antibodies were shown to be present in the piglets' circulations; in one case antibody was found in the piglet's circulation only 3½ hr. after suckling started, and by 4 hr. the titre of free antibody was 256. The antibodies did not pass the placenta.

In severe cases a rapid fall in haemoglobin, interrupted by a sharp, transient rise, and accompanied by reticulocytosis and erythroblastaemia, was characteristic of the first 24 hr.; this was followed in some cases by a prolonged low level of haemoglobin before recovery started.

In some of the animals that died, severe and widespread focal necrosis of the liver, together with degeneration and necrosis in the kidney, were marked. Pigment deposits and erythrophagocytosis were common in nearly all organs but abnormal extramedullary erythropoiesis was not observed.

A number of haematological, serological and biochemical findings are recorded.