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Amebic keratitis produced by Acanthamoeba spp. is an increasingly important ocular infection in extended-use contact lens wearers. Problems associated with the infection are compounded by the lack of effective and well-tolerated chemotherapeutic agents. The magainins, a group of naturally occurring and synthetic membrane-active peptide compounds, have been shown to be active in vitro against a clinical isolate of Acanthamoeba polyphaga. Two magainins tested extensively had minimal inhibitory and minimal amebicidal values of 20 and 25 micrograms/ml for magainin MSI-103 and 25 and 40 micrograms/ml for magainin MSI-94, respectively. Both amebastatic and amebicidal activities are enhanced by combining the magainins with silver nitrate (200 micrograms/ml) and/or other marginally effective antimicrobial agents. These combinations have activity against both trophic and cystic stages in the Acanthamoeba life cycle and have promise as antimicrobial agents in the treatment of amebic keratitis.