The findings of the present study, when combined with those of previous studies, support the following concept of binding of vitamin B12 by serum or plasma in vitro. There are primary binders of B12 which preferentially take B12 when small amounts are added and combine firmly with it. They include two already identified substances, TC I and TC II, which are of the seromucoid fraction of plasma. At concentrations of added B12 of the order of 1.0 ng./ml. of plasma, these proteins become saturated and secondary binders become important. The latter are relatively unimportant at lower concentrations of B12; they are weak binders of B12; they cannot be saturated by concentrations used to date, and they are not seromucoids.