We obtained four bacterial isolates from patients with Legionnaires' disease and examined them for in-vitro and in-vivo fine-structure characteristics. All isolates had an outer membrane, cytoplasmic membrane, and intracellular membrane structure. Numerous intracellular inclusions were seen, particularly from in-vivo specimens, and appeared membrane-limited. Fine-structure analysis did not reveal the presence of a definitive peptidoglycan structure. Isolation, purification, and chemical analysis of Legionnaires' disease bacterium peptidoglycan established molar ratios of alanine-glutamic acid and muramic acid-glucosamine. Diaminopimelic acid was absent in the Legionnaires' disease bacterium peptidoglycan. The Kellenberger procedure for fixation appears to be the best method for the fine-structure determination of Legionnaires' disease bacteria.