The host-parasite interactions between (i) macrophages and polymorphonuclear neutrophil leucocytes (PMN) of normal and immune mice, rabbits and guinea-pigs, and (ii) the facultative intracellular parasite, , were examined with the electron-microscope. Normal mouse cells were incapable of exerting a significant bactericidal effect and quickly succumbed to the necrotising action of the organism. Macrophages from immune mice were individually no more effective in killing , but the organisms disappeared from the peritoneal cavities more rapidly in immune than in normal mice. Bacterial clearance appeared to be associated with the reingestion by scavenging macrophages of viable within degenerating phagocytes. Suspending cells in hyperimmune sheep serum rendered them susceptible to bacteriolysis by normal or immune mouse cells. Rabbit PMN exerted an extremely rapid destructive action on the organism; rabbit macrophages and guinea-pig PMN and macrophages also destroyed but less effectively. In studies with all three animal species, viable organisms were found within phagolysosomes which were characterised by a relatively narrow peribacillary space and crenate limiting membrane. An electron-lucent zone always surrounded viable bacteria, and this was assumed to indicate the intact state of the surface lipid layer which was of similar density to the contiguous host enzymic material. Bacteria undergoing degeneration were often found within vacuoles with a wide peribacillary space and smooth limiting membrane; the surface lipid zone was wholly or partly obliterated in these organisms. It appears that surface lipid may function as a barrier against the degradative enzymes of mouse phagocytes.


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