1. Six cases of haemolytic disease in newborn foals apparently due to iso-immunization of pregnancy are described from their clinical, serological, haematological and pathological aspects. The findings agree closely with those seen in the same disease in newborn mules.
2. The disease may be diagnosed serologically by demonstrating the in vivo iso-sensitization of the foal's red cells by means of the direct antiglobulin-sensitization test. It is quite possible that after further investigation a modification of Diamond's albumin tube test may also be of value. Immune anti-red cell iso-antibodies of more than one specificity may be demonstrated in the dams' sera.
3. Despite the acute haemolytic process, very few reticulocytes and no erythroblasts are seen in the peripheral blood.
4. The morbid histology of the tissues of foals dying with the disease exhibits various points of interest which are discussed.
5. The clinical aspect is not described in this paper in any detail. However, a programme is suggested which would enable a successful exsanguino-transfusion of compatible blood to be carried out at very short notice. At present this seems to be the logical and most practical method of specific treatment.