1. Plasma samples from ten mammals were chromatographed on Sephadex G-200 columns and the total cobalamin content of each fraction was determined.
2. Unlike the situation in man, the bulk of the endogenous plasma cobalamin was found attached to a transcobalamin II-like protein in all ten animals. Transcobalamin 0 carried between 3 and 20%and this proportion appeared to be inversely related to the plasma total cobalamin. No endogenous cobalamin peak corresponding to transcobalamin I was detected in any species, though in the rabbit 5·3% of the plasma total cobalamin was attached to a protein of apparent molecular weight 176 000.