Summary. An elevated unsaturated vitamin B12 binding capacity (UBBC) had been observed in the sera of most subjects with leucocytosis of varying degrees and a few without evident leucocytosis. Although true alpha‐ and beta‐globulin binder increases of varying degree occurred, the increase in UBBC in most subjects was due mainly to a third vitamin B12‐binding protein. In addition, when UBBC fell as leucocytosis subsided, the third binder was no longer detectable. This protein resembles alpha‐globulin binder, or Transcobalamin (TC) I, in molecular weight, antigenic characteristics, and poor mediation of 57Co B12 uptake by reticulocytes. It resembles beta‐globulin binder, or TC II, in its behaviour on DEAE‐cellulose batch separation, electrophoretic mobility at pH 8.6, and, as suggested by indirect evidence, probably not carrying endogenous vitamin B12. Thus it appears identical to a serum binder previously thought to occur only in polycythaemia vera.It is apparent that the protein is present in large amounts in a variety of conditions involving leucocytosis, and possibly occurs in normal serum in small quantities also. It is possible that the binder may be an altered form of TCI. The reason for the predominance of TCI in some conditions involving granulocytic proliferation, and this binder in others, is unknown.