Rabbit liver homogenate, labelled in vivo with 57Co, was used to investigate the effect of anacid gastric juice on the release of cobalamins from food and on the cobalamin binding to proteins in humans. The cobalamin release was investigated in vitro by incubating the liver homogenate with native and neutralized gastric juice. The cobalamin release and the amount of cobalamins bound to R-protein were significantly higher with native than with neutralized gastric juice. To investigate the effect of anacidity on cobalamin release in vivo, 14 healthy volunteers were given omeprazole or placebo in a double-blind crossover fashion. 57Co-labelled liver-bound cobalamins were given orally, and aspirates were collected from the stomach and the upper jejunum. After omeprazole gastric anacidity (pH greater than 6.0) was obtained in 11, 12, and 10 individuals after 5, 30, and 60 min, respectively. In the gastric aspirates obtained after omeprazole there was a slight inhibition in cobalamin release after 5 and 30 min (p less than 0.05). In the jejunal aspirates the cobalamin release was close to 90% in both the placebo and the omeprazole experiment, and there was no significant difference in the percentage of cobalamins bound to R-protein in the two groups. The percentage of cobalamins bound to R-protein was much lower in the jejunal aspirates (26.6% and 23.7% in omeprazole and placebo, respectively) than in the gastric aspirates (65.8% and 68.6%).