SummaryIntestinal absorption of large oral doses of hydroxocobalamin and cyanocobalamin (100,. 500 and 1000 μg) was compared and quantitated in 17 normal subjects using a double label isotope technique.In 3 pairs of normal individuals, Co57-labeled cyanocobalamin and Co60-labeled hydroxocobalamin were administered simultaneously in oral doses of 100, 500 and 1000 μg and the ratio of the 2 isotopes in whole stool specimens was compared with that in the administered dose. No significant differences were found between the ratios administered and excreted at each dosage level. In 11 subjects intestinal absorption was quantitated by a modification of the double label hepatic uptake test in which the hepatic uptake of orally administered cyanocobalamin Co57 and hydroxocobalamin Co57 was given simultaneously with the first oral dose. At oral doses of 100, 500 and 1000 μg, mean hydroxocobalamin and cyanocobalamin absorption were not significantly different.