Plasma Vitamin P_12<12> in elderly and young subjects and the effect of oral administration of a multivitamin tablet on the plasma level were investigated. Both elderly and young groups were composed of 10 males and 10 females. Most of elderly had been suffered from cerebrovascular diseases, but they were able to live their daily lives without any supports, while young group was all healthy. Blood samples were collected before administration of the tablet, after the administration for 28 days, and at 8 days after the cessation. Plasma vitamin B_<12> was measured using radio-competition kit (magic, non-boil type). The results showed that marginal deficiency of vitamin B_<12> was found in 3 cases in elderly and in 3 cases in young subjects before the administration, that after the administration for 28 days, plasma B_<12> levels markedly increased in all groups, and that cases with marginal deficiency were all improved. Statistical tests revealed that the values significantly increased in elderly males (p < 0.05) and in young males (p < 0.01). It was found that the vitamin B_<12> plasma levels at 8 days after the cessation were slightly decreased in all groups and marginal deficiency reappeared in only one case of the same elderly subject. As for the relationship between protein intake and vitamin B_<12> plasma level, it was found that the higher protein intake brought higher plasma level of vitamin B_<12>.