Diverse Gram-negative bacteria communicate with each other by using diffusible N-acyl-homoserine lactone (AHL) signaling molecules to coordinate gene expression with cell population density. This mechanism termed 'quorum sensing' is involved in the regulation of physiological functions as well as multiple virulence determinants. It becomes more and more evident, that bacteria communicate not only with each other but also with their host. Up to now, little is known about this interkingdom communication. The AHL quorum sensing molecule N-3-(oxododecanoyl)-L-homoserine lactone (OdDHL) from Pseudomonas aeruginosa has been shown to influence the immune system of the host. The role and potential influence of other AHL molecules from other bacteria have so far not been determined. In this paper, we investigated the role of 7 different AHLs on apoptosis of human Jurkat T lymphocytes. We found, that among all homoserine lactones tested, only OdDHL rapidly induced apoptosis which was accompanied by the breakdown of the mitochondrial transmembrane potential (DeltaPsi(m)). Since overexpression of anti-apoptotic Bcl-2 completely abrogated the apoptotic effect, we presume that OdDHL induces apoptosis by activation of the intrinsic mitochondrial apoptosis pathway. The reason that bacteria induce apoptosis is largely unknown. We suspect that through apoptosis an anti-inflammatory response is triggered.