1. The metabolic responses to daily treatment with 1 μg cyanocobalamin were studied in sixteen patients with megaloblastic anaemia due to cobalamin deficiency.
2. The fall in serum iron was sluggish in most patients. The pattern of reticulocyte responses varied widely, the most common being a plateau with suboptimal maximal value.
3. Urinary propionic acid excretion before treatment was normal in two out of six patients. A transient rise during treatment was observed in two patients, possibly owing to demands on coenzyme stores. The fall in urinary propionic acid excretion during treatment was slow.
4. Serum vitamin B12 levels rose during treatment. In two patients an abrupt rise was found, possibly owing to release of stored cobalamin into the circulation.