Here we describe the first putative type I secretion system of L. pneumophila. The lssXYZABD locus was found to be present in all L. pneumophila strains tested so far. The LssB and LssD proteins are homologues to the proteins of type I secretion systems of Vibrio cholerae and Salmonella typhi. The expression of the gene locus was analysed by RT-PCR. Three polycistronic transcripts could be identified. Comparison of the lss locus of L. pneumophila Philadelphia I with that of L. pneumophila Corby revealed protein identities of 97 to 99%, except for LssE proteins that were only 78.6% identical. Computer analysis of the amino acid sequence of LssE identified it as a putative signalling protein.