Tissues from guinea pigs inoculated intraperitoneally with the Legionnaires' disease (LD) bacterium were studied with light, immunofluorescent, and electron microscopy. The principal gross lesion was diffuse peritonitis of varying severity. Microscopically, the peritoneum was covered by a mixed inflammatory infiltrate of macrophages, neutrophils, fibrin, and cellular debris. Foci of inflammation and necrosis were consistently observed in the splenic parenchyma, and similar lesions were often found in the lungs, liver, lymph nodes, pancreas, heart, and other organs. Numerous LD bacteria were seen in the peritoneal exudate; fewer were found in disseminated lesions. In electron micrographs, the highest concentrations were seen in macrophages, with fewer organisms present in neutrophils or extracellular spaces. Although the lung is the primary organ known to be affected by Legionnaires' disease in humans, our findings indicate that the LD bacterium is capable of dissemination.