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Since coenzyme B12 was fouud by Barker et al. (1) as an active form of vitamin B12 for the first time in 1958, its coenzymic activities have been indicated by many authors (2-8) but its metabolism has not been clarified. From the measurement using radioactive cyanocobalamin (CN-B12), and coenzyme B12 Okuda et al. (9) and Wong et al. (10) reported that the coenzyme had a greater affinity to liver and kidneys than CN-B12. On the basis of the measurement of tissue distribution and urinary and fecal excretion of those cobamides following oral administration, Uchino et al. (11) also indicated that coenzyme B12 had a greater affinity for tissue than CN-B12.Hydroxocobalamin (OH-B12) which was discovered first by Kaczka et al. (12), has attracted attention as one of the long acting derivatives for its retention in blood and affinity to tissues (13,14). It has also been regarded as a natural form of vitamin B12 occurring in the body, but little is known about the correlation between OH-B12 and coenzyme B12.In connection with the relationship between coenzyme B12 and intrinsic factor (IF) it was first suggested that IF had no effect on the absorption of the coenzyme (15), but later was indicated (16) that IF could enhance the absorption of the coenzyme from the gastrointestinal tract. Okuda et al. (17) described that IF administered intravenously with B12 could remarkably enhance the uptake of the vitamin by the liver. There is as yet very little information as to whether IF

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