1. The transcobalamins from normal serum were obtained in two fractions. One contained transcobalamin I and transcobalamin III: the other contained transcobalamin II. The forms of vitamin B12 in the two fractions were then examined.
2. Methylcobalamin and adenosylcobalamin were found in both fractions. Hydroxocobalamin was found in the fraction containing transcobalamin I and transcobalamin III. Cyanocobalamin was found in both fractions in two cases, in the transcobalamin III fraction only in one case and was absent in one case.