The clinical, pathological and serological findings for feline neonatal isoerythrolysis (FNI) in two litters are described. In each litter one of four kittens died shortly after birth, two showed varying degrees of depression, jaundice and haemoglobinuria and one remained healthy (clinically unaffected). Three of the kittens with jaundice died and the necropsy findings included variable degrees of erythrophagocytosis in the spleen and liver and many casts containing haemoglobin in the tubules and collecting ducts of the kidney. Blood typing and haemocompatibility testing were performed on the surviving kittens, queens and sires. Both queens and the healthy kittens were Type B while both sires and the surviving affected kitten were Type A. In both litters the queens sera haemolysed the sires erythrocytes but did not affect the erythrocytes of the healthy kittens. The erythrocytes of the surviving affected kitten were haemolysed by the serum of its queen. The pathogenesis and treatment of FNI are discussed.