Blood was drawn from the portal vein of rats immediately after the insertion of 65Zn into the duodenal lumen. Analysis of the plasma from the portal blood demonstrated that the major portion of absorbed 65Zn was bound to transferrin. When apotransferrin was incubated with 65Zn-labeled basolateral plasma membranes, 60% of the isotope was removed from the membrane. Zinc-free albumin prepared with histidine removed 25% of the 65Zn from labeled membranes and histidine had no effect. These results demonstrate that transferrin functions in the absorption and transport of zinc.