The postphagocytic effect on mouse, rabbit, and guinea pig peritoneal macrophages of a petrol-ether lipid extract from Corynebacterium ovis (C. pseudotuberculosis) representing the surface coat of the organism external to the cell wall was investigated by examing three parameters of cytotoxicity, viability assayed by dye exclusion, glycolytic activity, and ultrastructural morphology. The viability test demonstrated a lethal effect on normal and immune mouse macrophages but not on those of the rabbit or guinea pig. Measurement of glycolsis indicated a significant degree of cytotoxicity in normal mouse macrophages ingesting lipid, a nonsignificant depression of activity in cells from immune mice, and no alteration in the activities of rabbit and guinea pig macrophages. Electron microscopy demonstrated that C. ovis surface lipid caused acute lethal injury in normal and immune mouse macrophages. The early stages of degeneration were typified by dilatation of the cisternae of rough endoplasmic reticulum, Golgi lamellae, and nuclear envelope, proceeding to focal disruption of various cell membranes, particularly those of the lipidcontaining phagolysosomes and nucleus. In contrast, over the 3-h period of study, no cytotoxic change was evident in rabbit or guinea pig macrophages. The results add further support to previous observations that the surface lipid of C. ovis plays a major role in the pathogenesis of the organism in mice, but they do not explain the guinea pig's marked susceptibility to infection.