To make a food preparation containing radioactively labelled cobalamins, rabbits were given repeated injections with 57Co-labelled cyanocobalamin. The liver was removed, homogenized, and fried for 1 min or boiled for 30 min. Of the radioactivity in the fried homogenate 41.7% was recovered in the centrifuged supernatant compared with 50.8% in the boiled homogenate. The radioactivity in the supernatants had a molecular size close to that of free 57Co-labelled cyanocobalamin. Forty-two per cent of the radioactivity in the whole homogenate had been incorporated into 5-deoxyadenosyl-, 10% into methyl-, and 16.5% into hydroxy-cobalamin. To assess the validity of a double-isotope method for measuring the intestinal absorption of doses of the 57Co-labelled liver cobalamins, 51CrCl3 was used as a non-absorbable marker. In 14 healthy volunteers the correlation coefficient between the absorption measured by the double-isotope technique and the faecal excretion test was highly significant (r = 0.96, p less than 0.005), and there was only a small variation in the 57Co/51Cr ratio in successive stool collections. In 11 patients with chronic diarrhoea there was a significant correlation between the absorption measured by the double-isotope technique and the faecal excretion test (r = 0.92, p less than 0.005), but in some patients there was considerable variation in the 57Co/51Cr ratio in successive stool collections.