We have used light and electron microscopy to analyze endocytic trafficking pathways. In one set of studies, we have used fluorescently labeled antibodies to trace an endocytic pathway from the cell surface to the trans- Golgi network (TGN). Cells were transfected with a construct consisting of the transmembrane and cytoplasmic domains of TGN38 and the extracellular domain of Tac. TGN38 is predominantly in the TGN, but a small fraction is found on the cell surface. We used FITC-labeled anti-Tac monoclonal IgG to analyze the pathway from the surface to the TGN. We compared the distribution of internalized Tac-TGN38 to internalized transferrin. We found that most Tac-TGN38 enters the same early endosomes as transferrin. Furthermore, most Tac-TGN38 returns to the cell surface from the endocytic recycling compartment (ERC) at the same rate as transferrin. However, on each pass through the cell approximately 18% of Tac-TGN is retained, and this Tac-TGN38 is delivered to the TGN.