Elafin is a skin-derived serine-protease inhibitor. It is thought to be important to prevent human leukocyte elastase-mediated tissue damage and might play an important role in maintaining the integrity of the human epidermis. Recent studies have provided evidence for an antimicrobial activity of elafin against P. aeruginosa. As gram-negative infections typically occur in barrier-disrupted skin we were interested to determine whether supernatants of the gram-negative bacteria P. aeruginosa and Escherichia coli were capable of inducing elafin expression. Supernatants of various P. aeruginosa strains stimulated elafin mRNA-expression and protein release, whereas supernatants of E. coli did not induce elafin expression. In non-differentiated cells the relative increase of elafin mRNA was much higher (100-fold) than in differentiated cells (sixfold), although the latter exhibited higher constitutive mRNA-expression (150-fold). However, concentrations of secreted elafin were similar in differentiated and non-differentiated cells after stimulation. We could not confirm a bactericidal effect against P. aeruginosa as described previously but observed that its growth was inhibited as demonstrated for different strains in liquid cultures. Growth of E. coli was not affected by elafin. In conclusion, the data presented in this paper suggest that elafin represents an innate immune response factor induced by secreted products of P. aeruginosa. Besides its elastase inhibitory potency elafin is an antimicrobial agent against P. aeruginosa.