Primary amebic meningoencephalitis is a rapidly fatal infection caused by the normally freeliving ameba, Naegleriafowleri. It usually affects previously healthy children and young adults, who contract the disease by swimming in warm, stagnant, fresh-water ponds and other bodies of water inhabited by N. fowleri. The amebae reach the meninges via intranasal passage. At present, no reliable therapy is available for this disease. Among approximately 135 cases reported worldwide, there have been only three well-documented instances of survival. The most recent success was a 9-yr-old girl who was treated with amphotericin B and miconazole (both given intravenously and intrathecally) plus oral rifampin (Seidel et al., 1982, New Engl. J. Med. 306: 346348). Because of the poor prognosis for patients with primary amebic meningoencephalitis, a wide array of chemotherapeutic agents have been tested for in vitro activity against N. fowleri. Drugs shown to have in vitro efficacy include amphotericin B (Duma and Finley, 1976, Antimicrob. Agents Chemother. 10: 370-376); miconazole, tetracycline, and rifampin (Duma and Finley, 1976, loc. cit.; Thong et al., 1977, Lancet 2: 876); and clotrimazole (Jamieson, 1975, J. Clin. Pathol. 28: 446-449). We have performed sensitivity testing on a strain of N. fowleri using three previously untried agents: ketoconazole (Janssen Pharmaceuticals, New Brunswick, New Jersey); BAYn7133, a new investigational imidazole (Miles Laboratories, West Haven, Connecticut); and allopurinol riboside, an investigational antiprotozoal drug (Burroughs-Wellcome, Research Triangle Park, North Carolina). Amphotericin B, which has known activity against N. fowleri, was used for comparison (E. R. Squibb and Son, Inc., Princeton, New Jersey). A strain of N. fowleri (W.M.) isolated from a fatal human case was generously supplied by Dr. Richard J. Duma, Division of Infectious Diseases, Medical College of Virginia, Richmond, Virginia. Organisms were cultured axenically in calf serum-casein-yeast extract liquid medium (Chang's medium) (Chang, 1971, Curr. Top. Comp. Pathobiol. 1: 201-254) containing penicillin G, 100 U/ml, and streptomycin, 100 ,ug/ ml. To test drug susceptibility, an eight-chambered tissue culture chamber/slide system (LabTek?, Miles Laboratories, Naperville, Illinois) was used as described by Duma and Finley (1976, loc. cit.). Drugs were solubilized using sterile water, and then serially diluted in Chang's liquid medium to yield concentrations of 0.018, 0.078, 0.31, 1.25, 5.0, 10, and 50 ,g/ml. With an automatic, nonelectric pipette, 0.1-ml suspensions containing 105 trophozoites/ml, counted using a hemocytometer, from 72 to 96 hr (37 C), liquid, axenic cultures of N. fowleri were added to individual chambers containing 0.4-ml aliquots of each drug concentration. To one chamber per slide, as a control, amebae were added to axenic media only. Chamber/slides were incubated at 37 C. Minimal inhibitory concentrations (MIC), defined as the lowest concentration of drug at which growth was 50% or less than that of control, were determined at 24 and 48 hr by visual examination using an inverted microscope; values for each drug were the same at 24 and 48 hr. These results are summarized in Table I. To determine if growth inhibition was an amebastatic or amebacidal phenomenon, chambers showing inhibition of growth of amebae were re-subcultured into Chang's media, reincubated at 37 C for 48 hr, and observed for the presence of motile organisms. All subcultures failed to demonstrate viable amebae. Amphotericin B and ketoconazole both showed amebacidal effect on N. fowleri in vitro, whereas BAYn7133 or allopurinol riboside had no effect on the amebae at any of the concentrations tested. The MIC's for amphotericin B were lower than those reported previously by Duma and Finley (1976, loc. cit.), but were comparable to those