Purified human neutrophil elastase inhibits the growth ofE coli and K pneumoniae, but human cathepsin G does not. Mid-log phase bacteria (10⁵) were incubated in the absence (control) or presence of purified human neutrophil elastase or cathepsin G at 37°C for 4 hours, as described in Materials and Methods. Serial dilutions were immediately spread on agarose plates and the number of CFUs was determined after overnight incubation. Although neutrophil elastase inhibits the growth of both Gram-negative rods, cathepsin G does not. Neither enzyme effects the growth of S aureus. This experiment was repeated twice with similar results.