Legionella pneumophila is the causative agent of a severe form of pneumonia in humans (Legionnaires' disease). A major virulence factor, the Mip protein (FK506-binding protein, FKBP25mem), belongs to the enzyme family of peptidyl-prolyl cis/trans isomerases (PPlases). Here we show that L. pneumophila Philadelphia I possesses an additional cytoplasmic PPlase at a level of enzyme activity comparable to that of FKBP25mem. The N-terminal amino acid sequence of the purified protein was obtained by Edman degradation and showed that the protein is a member of the cyclophilin family of PPlases. The Icy gene (Legionella cyclophilin) was cloned and sequenced. It encodes a putative 164-amino-acid protein with a molecular mass of 17968 Da called L. pneumophila cyclophilin 18 (L.p.Cyp18). Amino acid sequence comparison displays considerable similarity to the cytoplasmic and the periplasmic cyclophilins of Escherichia coli with 60.5% and 51.5% identity, respectively. The substrate specificity and inhibition by cyclosporin A revealed a pattern that is typically found for other bacterial cyclophilins. An L. pneumophila Cyp18 derivative with a 19-amino-acid polypeptide extension including a 6-histidine tag and an enterokinase cleavage site exhibits PPlase activity when produced at high levels in E. coli K-12. After removal of the extension by enterokinase, the properties of the recombinant Cyp18 were indistinguishable from those of the authentic enzyme. In order to investigate the influence of Cyp18 on intracellular survival of L. pneumophila an Icy-negative L. pneumophila strain was constructed. Compared with the wild-type strain, the mutant did not exhibit a significant phenotype but was 10-fold less invasive for Acanthamoeba castellanii. Like human cyclophilin, the L. p. Cyp18 exhibits nuclease activity, but this enzymatic activity does not appear to be linked with the native structure of the protein.