In cases of anaemia in cattle and sheep, of infectious origin, parasitic origin, or due to simple haemorrhage, the phosphorus partition of the blood reveals an “unknown fraction,” confined to the corpuscles, and evidently nucleoprotein. It is determined by difference between the total phosphorus and the sum of total acid-soluble and lipoid phosphorus. It is associated with the appearance of nucleated red cells (normoblasts and Jolly bodies) and vanishes as they disappear. The amount may make up a considerable proportion of the total P.