1. The effects of the protein content of the diet on the vitamin B12 concentration of serum and of various tissues were studied in rats by means of microbiological and radioisotopic techniques.
2. Protein starvation reduced tissue weights and nitrogen contents, serum protein concentrations and haemoglobin values.
3. The vitamin B12 contents of the serum and of several organs and tissues were greater in the protein-starved animals for reasons that remain unexplained.
4. The retention of radioactive cyanocobalamin per g fresh weight was significantly greater in liver, kidney, spleen and brain in the protein-depleted rats than in those given adequate protein. However, the total tissue retention was not significantly different between these groups, so that there was a greater capacity for tissue binding and for body conservation of vitamin B12 in the protein-starved animals.