Naegleria fowleri amoebae were lysed by adult fresh human serum, and their multiplication was inhibited in culture medium supplemented with 10% fresh human serum. Heat inactivation (56 degrees C, 30 min) of serum abrogated these lytic and inhibitory effects. Absorption of human serum with amoebae failed to reduce immunoglobulin levels, and no specific antibody was detected in untreated or treated sera by counterimmunoelectrophoresis. Conversion of C3 and C3i occurred after incubation of n. fowleri with serum which had been treated with ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid, indicating activation of complement via the alternative pathway.