Washed red blood cells from normal cows were incubated as 10 and 20 per cent suspensions in media containing 0, 2.5 and 25 mM phosphate. The results showed that the rate of glycolysis was dependent on the inorganic phosphate concentration. In the absence of phosphate, the consumption of glucose and the production of 2,3-diphosphoglycerate, lactate and adenosine 5'-triphosphate (ATP) were decreased. Incubation without added phosphate also greatly increased the production of fruct0se-l,6,-diphosphate, glyceraidehyde-3-phosphate and di-hydroxyacetone phosphate. Moderate hypophospha-taemia was induced in two pre-ruminant calves. Washed red cells from the blood of these animals showed a depletion of ATP when compared with red cells from a control calf. The results indicate that phosphorus deficiency, leading to hypophospha-taemia, may be a mechanism of post parturient and related syndromes of haemoglobinuria by decreasing red cell glycolysis and resultant ATP synthesis. Sub-normal concentrations of ATe would predispose red cells to altered structure and function, a loss of normal deformability, and an increase in fragility and haemolysis with resultant haemogiobinuria.