Cercropin A (CA), a boactive peptide, proved toxic to Enterobacter agglomerans (EA). Concentrations of 0.125 and 0.025 M gave 100 and 75% mortality, respectively. Significant mortality occurred in as little as 20 min post addition of CA to the bacteria. Preincubation of CA with EA lipopolysaccharide (LPS, 1 mg/ml) completely blocked CA killing of EA. Palmitic and myristic acids, components of this LPS, individually at 1 mg/ml. did not inhibit toxicity. However, trilinolein (l mg/ml), did reduce CA toxicity by 49%, while diphosphoryl lipid A (a mg/ml) from two different bacteria eliminated CA toxicity to EA. The data indicates that CA binds with the diphosphoryl lipid A moiety of LPS. We believe this is the first report of CA interacting with LPS.