The deaths on nine farms of lambs which had been fed cow colostrum as a substitute for ewe colostrum were investigated. Of 105 lambs which received cow colostrum, 65 (61.9 per cent) showed clinical signs of anaemia and 42 (40 per cent) died. The signs of anaemia usually appeared when the lambs were between eight and 12 days old. The most significant post mortem finding was the appearance of the bone marrow which was cream or grey rather than the normal bright red. The types of treatment which were given are summarised. Whey from samples of the colostrum fed to the lambs was tested for its effect on sheep red blood cells. Haemolysis or agglutination of the red cells occurred with some, though not all, of the samples which caused anaemia.