EXTRACT Quorum sensing is a cell-to-cell communication system used by pathogenic bacteria to control expression of virulence factors. In Pseudomonas aeruginosa, quorum-sensing mutants show reduced virulence and, in a recent issue of PNAS, Chun et al. reported that human respiratory epithelia have the capacity to inactivate a P. aeruginosa quorum-sensing signal. This capacity appears to be enzymatic in nature, and it functions in some but not all mammalian cells. This finding opens a new area of research and indicates that humans have evolved mechanisms to interfere with a quorum-sensing pathway.