The chemotactic and tumoricidal properties of the pathogenic/free-living amoeba Acanthamoeba castellanii were examined in vivo and in vitro. A. castellanii trophozoites displayed strong positive chemotactic responses to human melanoma (OCM-1) and murine melanoma (D5.1G4) cells. Although the parasites typically invade and destroy the corneal epithelium and stroma, positive chemotactic responses were not detected against extracts of either of these corneal elements. In vitro studies revealed that viable parasites, as well as cell-free parasite lysates, produced swift and extensive cytolysis of a wide variety of tumor cells. The tumoricidal properties of A. castellanii were examined in vivo and revealed that injection of viable parasites or cell-free parasite lysates into progressively growing subcutaneous melanomas resulted in 83% and 53% reductions in the tumor masses compared with untreated controls. The feasibility of utilizing the tumoricidal properties of pathogenic/free-living amoebae and their cell-free products in the treatment of drug-resistant or radioresistant tumors warrants further investigation.