Wistar rats bearing N-nitrosomethylurea-induced mammary tumors received intratumoral injections of prospidium chloride or saline. Each tumor with 30-100 mm2 (the product of the two largest perpendicular diameters) received 20 mg of prospidium chloride diluted in 100 microliter of saline, and increasing amounts of 20 mg of the product in the same volume for every additional 100 mm2 or fraction of tumor. After a week the treatment was repeated in the same way. Control rats received only saline without prospidium chloride. No significant differences were observed regarding the tumor area, total tumor area/rat, and body weight when comparing the rats receiving prospidium chloride and the control rats. Prospidium chloride does not show any direct oncolytic effect upon the experimental model employed.