Legionella pneumophila is an intracellular parasite which is able to survive and multiply in human monocytes and alveolar macrophages. The Mip (macrophage 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 PPIases of eukaryotes. However, the N-terminal extension of Mip and/or the substitutions of the variable amino acids in the C-terminal FKBP core leads 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.