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Serum Vitamin B12 levels were determined on 378 patients ranging in age from 56 to 104 years with a median age of 77 for both males and females. A radiodilution method was employed for these determinations. This screening procedure identified 26 patients with low serum B12 levels. Nineteen of these patients had no other symptoms and were hematologically normal. The B12 binding proteins, transcobalamins (TC) I, II, and III were quantitated employing QUSO and DEAE cellulose batch separations. The total number of TC II binding/affinity sites for B12 were elevated in both the normal and low B12 elderly groups. About 17% of the total serum B12 was carried by TC II in the control group while only 4% of the total was carried on TC II in both the normal and low B12 elderly. This was accompanied by an increase in unsaturation in TC II for these two groups. The findings suggest that an alteration in the TC II-B12 delivery system has occurred in the elderly.