These studies showed that increased natural killer (NK) cell activity develops during experimental American Trypanosomiasis in mice. Increased cytotoxicity against YAC target cells by peritoneal cavity cells (PEC) was detected as early as 1 day into infection in both C3H/He and C57BL/6 mice. Peak activity occurred 2 days into infection, with significant but variable increases detected in the spleen cells (SC) from these mice. The response subsided in PEC and to a lesser extent in SC by 4 days post-infection. Similar responses were not detected in infected C57BL/6 bgJ/bgJ (beige mutant) mice until the fourth day of infection and peaked at significantly lower levels than the peak on day 2 in beige hybrid mice ([C57BL/6 +/+ X C57BL/6 bgJ/bgJ]F1). The effector cell from infected mice was sensitive to pretreatment with anti-NK 1.2 serum plus complement (C), partially sensitive to anti-Thy 1.2 + C, and insensitive to polyvalent anti-mouse immunoglobulin + C. The cytotoxic activity induced in cells from mice infected for 2 days was recovered in subpopulations nonadherent to plastic or Sephadex G-10. Thus, the anti-YAC effector cell found very early in mice infected with T. cruzi possessed many of the characteristics of natural killer (NK) cells found in normal mice. Other experiments demonstrated that injection of heat-killed preparations of blood-form trypomastigotes (BFT) induced increases in NK activity. The presence of augmented NK activity against YAC tumor cells in both resistant and susceptible strains of mice, together with the presentation of the resistance phenotype in beige mutant mice, which had a lower NK response to infection, is not indicative of a direct role for host protection during infection with T. cruzi by NK cells.