Amebas of the genus Naegleria have been shown to harbor an agent, recognized by its ability to produce cytopathic changes in a variety of tissue culture assay systems and, possibly, mice. This agent, in conjunction with thermal tolerance of the ameba, may be a factor in determining pathogenicity of Naegleria fowleri for humans, as the causal agent of primary amebic meningoencephalitis. Cytopathology in tissue culture assay systems will be studied employing labeled precursors and electron microscope autoradiography. Attempts will be made to separate and characterize the active agent in ameba lysates using acrylamide gel electrophoresis. Finally, lysates and fractions derived from them will be tested on experimental animals for evidence of ability to produce symptoms similar to those seen in animals infected with whole amebas. BIBLIOGRAPHIC REFERENCES: Schuster, F.L., & Rechthand, E. 1975. In vitro effects of amphotericin B. on growth and ultrastructure of the amoeboflagellates Naegleria gruberi and Naegleria fowleri. Antimicrobial Agents & Chemotherapy 8, 591-605. Schuster, F.L. 1976. Fine structure of the schizont stage of the testate marine ameba, Trichosphaerium sp. J. Protozool., in press.