Plasmid and peptide analysis was used to characterize Legionella pneumophila strains isolated in the study of a small cluster of cases in hospitalized patients. The isolates from the Denver Veterans' Administration Medical Center could be clearly separated into three groups. Two of the three clinical isolates were found to be plasmidless, as were five of 19 environmental isolates. The patient isolates had plasmid and peptide profiles which were identical to the showerhead isolates to which each patient was exposed. Thus, the data suggest that the patients acquired their disease strains from environmental sites in their particular hospital wing, and that each wing of the building had its own unique flora of Legionella strains. The results also confirm the usefulness of using both these techniques when tracing transmission patterns of nosocomial disease.