Young rats were fed diets containing 1.44% cereal protein (wheat and rye), 3.65% salt mixture, 2.9% fish liver oil, 1.46% brewer''s yeast, and 78% carbohydrate (starch). Four groups of 30-50 rats each received the following supplements to increase the protein conc. in the diet to 7-9%; A, fish meal; M, skim milk powder; B, brewer''s yeast; B II, wood sugar yeast. A and M showed good growth, B and B II half as much. Addition of 0.2% of cystine to the B diet gave growth as good as A and M, and decreased the mortality of the B rats. The livers of the B rats had a mottled appearance, were much enlarged and friable and, histologically, showed fatty degeneration of the capillary endothelium and liver cells, with occasional necrosis; the conc. of glycogen and fat and the dry wts. were lower than for normal livers. The kidneys showed degenerative changes and evidence of glomerulonephritis. Cystine appeared to have a specific, vit.-like importance.