The potential role of inhaled protozoa as a niche for intrapulmonary replication of Legionella pneumophila was investigated in vivo with mutant strains of L. pneumophila which have reduced virulence for the amoeba Hartmannella vermiformis. L. pneumophila AA488 and AA502 were derived from wild-type strain AA100 after transposon mutagenesis. These mutants have reduced virulence for H. vermiformis but are fully virulent for mononuclear phagocytic cells. A/J mice, which are susceptible to replicative L. pneumophila lung infections, were inoculated intratracheally with L. pneumophila AA100, AA488, or AA502 (10[6] bacteria per mouse) or were coinoculated with one of the L. pneumophila strains (10[6] bacteria per mouse) and uninfected H. vermiformis (10[6] amoebae per mouse). The effect of coinoculation with H. vermiformis on intrapulmonary growth of each L. pneumophila strain was subsequently assessed. In agreement with our previous studies, coinoculation with H. vermiformis significantly enhanced intrapulmonary growth of the parent L. pneumophila strain (AA100). In contrast, intrapulmonary growth of L. pneumophila AA488 or AA502 was not significantly enhanced by coinoculation of mice with H. vermiformis. These studies demonstrate that L. pneumophila virulence for amoebae is required for maximal intrapulmonary growth of the bacteria in mice coinoculated with H. vermiformis and support the hypothesis that inhaled amoebae may potentiate intrapulmonary growth of L. pneumophila by providing a niche for bacterial replication.