A morphometric analysis was made to study membrane traffic in bone marrow-derived macrophages, containing phagosomes with partially degraded Bacillus subtilis. Cell surface glycoproteins, labeled with radioactive galactose by terminal glycosylation, provided a covalent autoradiographic membrane marker. Membrane compartments were characterized in terms of cytochemical staining for horseradish peroxidase taken up by receptor-mediated endocytosis. The area, composition, and exchange rates of endocytic membrane compartments were measured as in a previous analysis for non-infected macrophages, devoid of phagosomes. In direct comparison with this earlier study, the present data allowed an assessment of the involvement of phagosomes in the interactions between endocytic membrane compartments. The presence of phagosomes led to a 30% reduction of lysosomal membrane area. The rate at which cell surface-derived label flowed into the lysosomal membrane pool was reduced by the same fractional amount. This suggested a linear relationship between flow rate and membrane area. The initial flow rate of label into phagosomes was higher than expected, based on their membrane area being only about 60% that of lysosomes. This rate could only be measured during the early phase of the experiments when phagosomes were younger, therefore displaying a fast exchange rate, reminiscent of the endosome compartment. However, steady-state conditions, at late times, strongly suggested that phagosomes with degraded contents finally acquire membrane of lysosomal origin. First, the composition of phagosome membrane became the same as that of lysosomes, remaining unchanged as compared to non-infected cells. Second, the membrane area of phagosomes amounted to the loss of lysosomal membrane area in infected cells.