Endemic nosocomial Legionnaires disease has occurred at Ohio State University Medical Center for several years. Two subtypes of Legionella pneumophila serogroup 1 (UH-1 and RH-1) have been isolated in approximately equal numbers from hospital potable water. However, almost all clinical isolates have been UH-1. To assess potential difference in virulence, 50% lethal doses (LD sub 50) and 50% infective doses (ID sub 50) of UH-1 and RH-1 were determined by intraperitoneal infection in guinea pigs. The UH-1 LD sub 50 was 7.41 x 1,000,000 CFU, which was significantly lower than the RH-1 LD sub 50 of 9.12 x 10 to the 7th power CFU (P = 0.0001). The mean time to death in UH-1-infected guinea pigs was also significantly shorter than in RH-1-infected animals (P=0.0008). The UH-1 ID sub 50 of 1.4 x 10,000 CFU, this difference was not statistically significant (P=0.21). This study demonstrates a difference in virulence between UH-1 and RH-1 in guinea pigs. Differences in strain virulence, as demonstrated between these two subtypes, may help to explain the widespread isolation of L. pneumophila from the environment contracted with the limited occurrence of human Legionnaires disease. (Copyright (c) 1985, American Society for Microbiology.)