To examine the effect of dietary protein on vitamin B_<12> (B_<12>)-deficiency, rats were fed on four different types of B_<12>-deprived diets: (1) 8% casein diet (8%), (2) 18% casein diet (18%), (3) 40% casein diet (40%), and (4) 18% soy bean protein diet (SP). Growth retardation of B_<12>-deprived rats fed on a 40% diet was the severest of all the rats fed on four experimental diets. The 8% diet and SP diet groups did not show any difference in the degree of growth retardation due to B_<12>-deprivation for 140 days, as compared with the 18% diet group. The increase in urinary excretion of methylmalonic acid (MMA) upon B_<12>-deprivation was the most prominent in the 40% diet group among all four diets, and MMA was proportionate to amount of protein level, although urinary MMA was not influenced by protein quality. The increase in the tissue weight-to-body weight ratios of several organs was more prominent in the 40% diet group than in the 18% diet group. Decreases in the tissue B_<12> concentration of B-_<12> deprived rats fed on the 8%, 18% and SP diets for 140 days, and fed on a 40% diet for 90 days, showed the same tendency among several tissues.