Mouse peritoneal macrophages were cultured in vitro, exposed to exogenous tracers, and examined by transmission electron microscopy. Native, anionic ferritin and horseradish peroxidase did not bind to the cell surface and were exclusively found in endocytic vesicles and lysosomes after uptake into the cells. Cationized ferritin (CF) bound to the plasma membrane and was then rapidly internalized. Intracellularly, CF first reached the lysosomes, but was later also transferred to and passed through the Golgi complex. These findings tentatively suggest that content and membrane of endocytic vesicles partly may follow different routes within macrophages. The content is emptied into lysosomes, from which part of the incoming membrane subsequently can be detached and moved over to the Golgi complex for reutilization. Additional, indirect evidence in support of this idea was obtained in double-labeling experiments with latex beads and CF. Thus, as expected with regard to the persistent nature of the latex-containing phagolysosomes, the transfer of CF to the Golgi complex was of a much smaller magnitude in cells exposed to both tracers than in cells exposed to CF alone. Pretreatment of the macrophages with colchicine removed all cytoplasmic microtubules and led to a characteristic disorganization of the Golgi complex. The treatment did not interfere with the binding of CF to the plasma membrane, however, but distinctly inhibited its uptake and transport to the Golgi complex. Contrarily, lumicolchicine and cytochalasin B did not affect the uptake and intracellular destination of CF. These results confirm previous notions concerning a role of cytoplasmic microtubules in endocytosis. They further indicate that absence of microtubules affects the intracellular handling of molecules binding to the cell surface.