An association between brain cysticercosis and malignant neoplasms in humans has recently been reported. To explore the possibility of a potentiating effect of cysticercosis on carcinogenesis mice infected with Taenia crassiceps cysticerci were exposed to the carcinogenic substance methyl-nitrosourea; 35% of them developed lymphoma, in contrast with 50% of control non-infected animals exposed to MNU. In this experimental model of cysticercosis we did not find a potentiating effect of peritoneal cysticercosis on the carcinogenicity of MNU.