Lambs grazing cobalt-deficient pastures and injected with hydroxocobalamine gained significantly more weight and excreted significantly less methylmalonic acid in the urine than untreated controls. Lambs with liver vitamin B12levels in the range 0.1–0.2ώg/g excreted less than 25 ώg of methylmalonic acid per ml of urine, whereas lambs with liver vitamin B12concentrations of less than 0.1 ώg/g excreted greater amounts. Lambs in both groups had serum vitamin B12concentrations less than 500 pg/ml.