Rat livers were perfused in situ with [¹²⁵I]-transferrin, FITC-dextran and [³²P]-inorganic pyrophosphate (PP). In a parallel set of experiments rat liver homogenates was incubated with [¹²⁵I]-transferrin and [³²P]-PP_i. In both sets of experiments cellular organelles were prepared by density gradient centrifugation in metrizamide. In the perfusion experiments [¹²⁵I]-transferrin and FITC-dextran accumulated in fractions rich in plasma membrane enzymes, whereas in the homogenate experiments [¹²⁵I]-transferrin remained in soluble fractions. [³²P]-PP_i remained in soluble fractions in both sets of experiments, i.e. PP_i did not co-localize with transferrin taken up by intact cells. In another set of experiments fractions rich in endosomes were tested for their ability to synthesize PP_i from ATP. No PP_i was found. The results argue against a role for PP_i in transferrin-iron release within endosomes.