Legionella pneumophila is an intracellular parasite which is able to survive and multiply in human monocytes and alveolar macrophages. The Mip (macro-phage infectivity potentiator) protein has been shown to be an essential virulence factor. A search of translated nucleic acid data bases has shown that the Mip protein from strain Wadsworth possesses regions homologous to those found in the FK506-binding proteins (FKBPs) of several different eukaryotic organisms. FKBPs are able to bind to the immunosuppressant macrolide FK506 and possess peptidyl-prolyl cis/trans isomerase (PPIase) activity. The gene coding for the Mip protein was cloned from the chromosome of L. pneumophila strain Philadelphia I and sequenced. It was synthesized in Escherichia coli K-12 and after purification it exhibited PPIase activity catalysing the slow cis/trans isomerization of prolyl peptide bonds in oligopeptides. Mip is inhibited by FK506 and fully resistant to cyclosporin A, as was also found for the recently characterized FKBP-type PPlases of eukaryotes. However, the N-terminal extension of Mip and/or the substitutions of the variable amino acids in the C-terminal FKBP core lead to variations, when compared with eukaryotic FKBPs, in substrate specificity with the oligopeptide substrates of type Suc-Ala-Xaa-Pro-Phe-4-nitroanilide. Nevertheless, the Legionella Mip factor represents a bacterial gene product which shares some characteristics normally found in eukaryotic proteins. In view of the activity of PPIases in protein-folding reactions, such prokaryotic FKBP analogues may represent a new class of bacterial pathogenicity factors.