Human intrinsic factor (IF) saturated with (60)Co-labeled cyanocobalamin ((60)CoB(12)) was purified and then iodinated with (125)I to yield (125)I-labeled IF-(60)CoB(12) preparations of high specific activity. Sephadex G200 and DEAE-cellulose chromatography of the iodinated IF-(60)CoB(12) complex showed coincidence of the major (125)I and the (60)Co radioactivity peaks. During starch-gel electrophoresis (60)Co radioactivity from noniodinated and iodinated complexes migrated to the same extent while (125)I radioactivity from the iodinated complex migrated slightly further anodally than did the (60)Co radioactivity. After the iodinated complex was mixed with antibody to the IF-B(12) complex (antibody II) the (125)I and (60)Co radioactivity were: (a) precipitated in similar amounts by antiglobulin serum. (b) eluted coincidentally in the 19S region on Sephadex G200, and (c) excluded to the same extent from starch gel during electrophoresis. After equilibrium exchange of IF "blocking" antibody (antibody I) for (60)Co-vitamin B(12) on (125)I-labeled IF. (125)I radioactivity from the IF-antibody I complex: (a) was precipitated by antiglobulin serum, (b) was eluated in the 19S region on Sephadex G200 gel filtration, and (c) migrated slowly towards the anode on starch-gel electrophoresis. Urinary excretion of (60)Co radioactivity in pernicious anemia patients after oral administration of (60)Co-vitamin B(12) bound to freshly prepared (125)I-labeled IF was similar to that obtained with noniodinated intrinsic factor. These results show that iodination of IF-(60)CoB(12) complex does not markedly alter the chromatographic, electrophoretic, antigenic, or absorption-promoting properties of IF.