Weaned rate were used to Investigate the aetiology of the anaemia associated with protein deficiency. In preliminary experiments, groups of rats were maintained on diets of four different protein contents (2, 3, 5 and 10 NDpCal^) and assessments were made of their general characteristics, of the degree and nature of anaemia and of serum proteins. It was found that the severity of anaemia correlated well with the protein content of the diet and also varied with the duration, becoming severest towards the end of the growing period ( about week 8 ) and then gradually improving. Similar patterns were observed for the serum proteins end other characteristics. The anaemia was of mild or moderate degree and invariably of normocytic normochromic type. Hyperplasia of the bone marrow was apparent from normoblast counts, yet reticulocyte counts were in the normal range. Serum transferrin was reduced but this appeared to play little part in the aetiology of the anaemia. Further investigations were performed on rats maintained on the 2 and 10 NDpCalS- diets for 8 weeks. Erythrocytes from the protein deficient rats were observed to suffer more rapid haemolysis in control receiver rats than those frem protein replete animals, while their osmotic fragility we reduced. The plasma erythropoietin level was elevated in the protein deficient rats, in fact higher that in rats on control diet bled to the same degree of anaemia. It was concluded that the primarey cause of the anemia was an insufficiancy of protein supply at the bone marrow. The elevated erythropoietin level stimulated the bone marrow into hyperplasia but was unable to prevent the onset of anaemia. Extravascular haemolysis was a secondary cause, reflecting capture by the spleen of defective erythrocytes released from the bone marrow, but being insufficient on its own to account for the observed degree of anaemia.