Five different forms of transferrin (rat apo [iron-free], rat diferric, diferric rat asialo, human diferric, and diferric human asialotransferrin type 3) were used to monitor the passage of this protein and its metal to the bile. Cumulative biliary excretion of the dose over 3 hours was determined. In addition, an excretion profile was constructed from the concentration of tracer in bile samples collected over 10-minute intervals. The profile obtained with apotransferrin was very similar to that found in an earlier study with albumin, the implication being that the apo form is transferred passively (e.g., by diffusion). Behavior of rat diferric transferrin, however, was consistent with the assumption that this form is transferred both passively and actively (i.e., in vesicles). The three other transferrins were investigated with the intent of broadening the spectrum of ligand affinities for the plasmalemma of hepatocyte. The higher this at- traction was, the larger fraction of the dose appeared in bile. When transferrin was targeted to Iysosomes, the bile contained several intermediate proteolytic fragments. Double-labeled (lZ5I, 59Fe) transferrin was used to measure recovery of iron (Fe) relative to the protein (P) in bile. With rat diferric transferrin, the Fe/P ratio was 0.72. Lower values were recorded with transferrins (human or asialo) that had higher affinities for the plasmalemma and therefore were expected to be transported to a larger extent in vesicles. Of the biliary 5 % ' e , 85% to 92% was protein bound. The proportion of the protein-bound fraction was essentially independent of the magnitude of FeP ratios. (HEPATOLOGY 1995;21:1063-1069.) Proteins derived from the circulating plasma feature high among the normal constituents of and interest in tracking the way(s) they get there has prompted numerous investigations. As the combined result of these efforts, it now seems certain that several transport mechanisms are involved. One group of them, conveniently captioned as passive routes, encompasses both molecular weight-dependent and -inde-From the