The effect of Gladenine, an oncolytic antibiotic produced by a newly discovered fungus, Paecilomyces todicus, nov. sp. was investigated on Harding-Passey, hard tumor, melanoma. A long-term screening technique was used in the investigation. 15 mg/100 gm. wt/mouse was injected I.P. starting 10-14 days after transplant, when all tumors were well pronounced, and continuing for 18-34 days. The majority of mice did not lose weight. Tumors were absent on the average in about 33% of the mice and average T/C was .16. Combining Cladenine with Hydrazine sulfate, either 1.5 mg. or 2.5 mg./100 gm. wt./mouse, using the same screening technique, resulted in 60-100% of mice free of tumors, and still remaining so several months later. Average T/C was .06. Combined therapy with Gladenine and Hydrazine sulfate at 3.75 mg./100 gm. wt./mouse gave less satisfactory results. Repeated screening tests using the same long term techniques, gave similar or identical results. It appears that Harding-Passey melanoma is susceptible to the chemotherapy of Gladenine alone and/or combined with small doses of Hydrazine sulfate.