Cocultivation of Acanthamoeba castellanii and Acanthamoeba polyphaga with live Pseudomonas aeruginosa and with broth filtrates of P. aeruginosa proved equally lethal to the Acanthamoeba spp. The P. aeruginosa-induced amebicidal activity is apparently toxin mediated and has two operative modes: it can function through binding of P. aeruginosa to the ameba membrane and in the presence of one or more P. aeruginosa exoproducts.