⁵⁷Co-labelled vitamin B₁₂ (0.25 µg; 0.25 µCi) has been used in a double tracer test of absorption, together with ⁵⁸Co-labelled vitamin B12 (0.4 µg; 0.4 µCi), using a whole-body monitor. It was found that variation of the counting-rate on the day of administration was small and was similar for both isotopes. The fraction of ⁵⁸Co γ-rays detected in the ⁵⁷Co energy band also showed little variation and the patient was able to act as his own phantom. The range of absorption of vitamin B₁₂ was 21.3-63.3% in control subjects and 1.8-11.8% in patients with pernicious anaemia. The absorption of vitamin B₁₂ with intrinsic factor, determined simultaneously, was always at least twice the absorption of vitamin B₁₂ alone in patients with pernicious anaemia and always less than twice in control subjects. The results demonstrate the suitability of ⁵⁷Co-labelled vitamin B₁₂ in whole-body monitoring studies of vitamin B₁₂ metabolism.