The ink of sea hares (Aplysia californica) contains escapin, an L-amino acid oxidase that metabolizes L-lysine, thereby producing a mixture that kills microbes and deters attacking predators. This secretion contains H2O2,ammonia, and an equilibrium mixture of "escapin intermediate product" (EIP-K) that includes alpha-keto-epsilon-aminocaproic acid and several other molecules. Components of the equilibrium mixture react nonenzymatically with H2O2 to form "escapin end product" (EEP-K), which contains delta-aminovaleric acid and delta-valerolactam. The proportions of the molecules in this equilibrium mixture change with pH, and this is biologically important because the secretion is pH 5 when released but becomes pH 8 when fully diluted in seawater. The goal of the current study was to identify which molecules in this equilibrium mixture are bactericidal. We show that a mixture of H2O2 and EIP-K, but not EEP-K, at low mM concentrations is synergistically responsible for most of the bactericidal activity of the secretion against Escherichia coli, Vibrio harveyi, Staphylococcus aureus,and Pseudomonas aeruginosa. Low pH enhances the bactericidal effect, and this does not result from stress associated with low pH itself. Sequential exposure to low mM concentrations of EIP-K and H2O2, in either order, does not kill E. coli. Reaction products formed when L-arginine is substituted for L-lysine have almost no bactericidal activity. Our results favor the idea that the bactericidal activity is due to unstable intermediates of the reaction of alpha-keto-epsilon-aminocaproic acid with H2O2.