Mice infected with Trypanosoma cruzi were challenged with 2x10[raised to the power of 6] cells of sarcoma 180 (ascite tumor) by i.p. route, on day seven post infection. Tumor development was followed by evaluation of weight gain, by measurement of ascitic fluid produced and enumeration of tumor cells in ascitic fluid. Infected mice were more resitant to tumor development as demonstrated by reduction in ascites formation and by reduction in the number of tumor cells in ascitic fluid, at different time intervals after tumor challenge. The number of peritoneal cells exsudated after tumor inoculation was greater in infected mice than in controls. This increased resitance of mice infected with T. cruzi to tumor development could be due to the action of macrophages activated by the infection and by the action of endotoxins absorbed from the gut or produced by the own parasite.