The uptake of iodinated human intrinsic factor by guinea pig ileum was studied in vivo using electron microscopy radioautography. Iodination of intrinsic factor cobalamin complex with N-chlorobenzene-sulphonamide beads did not modify its physicochemical properties in gel filtration, nor in iso-electrofocusing. It didn't affect its biological activity either in vitro in presence of solubilised receptor, or in vivo in the Schilling test. The 125I-intrinsic factor cobalamin complex was instilled in vivo in ileal blind loops in presence of either CaCl2, EDTA, bile or intestinal juice. The labelled complex was predominantly located in the intermicrovillous pits of the apical membrane and in the apical cytoplasm. The uptake was about five-fold lower in presence of EDTA. On the apical membrane, the number of grains (per 80 micron length of epithelium) increased from 2.7 (0.2) up to 9.4 (1.7) respectively for 15 minutes and two hours of delay. This suggests a recycling of the intrinsic factor receptor complex. In the apical compartment, the silver grains were often detected over non-coated vesicles and over the infoldings of the lateral membrane. These results show that intrinsic factor is internalised into enterocytes during cobalamin absorption, and that a part of intrinsic factor enters the blood circulation through transcytosis.