1. The long-term clearance of radioactive vitamin B12 from serum and from whole body was measured in control subjects and patients with treated pernicious anaemia and the data were analysed by the occupancy principle to provide estimates of dietary intake, daily requirements and whole-body mass of vitamin B12.
2. In six cases the daily requirement for vitamin B12 ranged from 0·15 to 1·9 μg with an average of 1·0 μg. The whole-body vitamin B12 in four cases ranged from 780 to 1350 μg with an average of 1060 μg.
3. Estimates of the whole-body vitamin B12 on the assumption of single-pool equilibrium gave constantly changing results, which, together with discrepant half-lives for serum radioactivity and whole-body radioactivity, constitute evidence against the concept of single-pool equilibrium.