Sodium dodecylsulfate--polyacrylamide gel electrophoresis has been employed to analyze the protein and glycorprotein components of phagosome membranes prepared from mouse L cells by the polystyrene latex bead method. 2. These experiments showed that phagosome membranes contain at least 20 major membrane protein species having apparent molecular weights between 27,000 and 250,000; the most abundant proteins have molecular weights of 112,000, 103,000, 76,000, 68,000, 62,000, 42,000 and 36,000. 3. Phagosome membrane glycoproteins, which were detected on the gels by staining with periodic acid-Schiff's reagent, were found to migrate in two broad zones centered at positions on the gel corresponding to proteins of mol. wt 140,000 and 85,000. 4. Comparison of the phagosome membrane results with the results of similar experiments carried out with cell surface membranes revealed a high degree of similarity between the two. All major protein and glycoprotein components present in phagosome membranes could be identified in both of two types of cell surface membrane preparations analyzed. 5. These results strongly support the view that phagosome membranes contain a representative, not a highly selected, sample of surface membrane proteins and glycoproteins.
