The intraphagosomal survival strategy of pathogenic mycobacteria was studied in bone marrow-derived mouse macrophages. These bacteria survive inside phagosomes by interfering in an unknown manner with phagosome processing which normally would lead to digestion of the phagocytic particle in phagolysosomes. Here, phagosome processing was compared for different phagocytic particles: live Mycobacterium avium, degradable Bacillus subtilis, or indigestible latex beads. We show detailed electron microscopic morphological observations which characterize various phases of interaction between endocytic organelles and phagosomes. We measured fusion of phagosomes with early endosomes or with lysosomes by using newly internalized endocytic contents (horseradish peroxidase, HRP) and membrane marker (plasma membrane glycoconjugates labeled with [3H]galactose via exoglycosylation). Morphometric analysis of these observations showed that the nature of the phagocytic particle affects phagosome processing: As long as particles remain undigested, maturation of phagosomes is prevented and they remain fusogenic towards early endosomes; concurrent to particle digestion, phagosome processing proceeds towards transfer of phagocytic contents to phagolysosomes which display kinetic and compositional characteristics of lysosomes. As an intact phagocytic particle, M. avium remains in non-matured phagosomes which fuse with early endosomes, but not with lysosomes. Fusion with early endosomes is reduced, thereby indicating the stage where this endoparasite exerts its effect.