Adult MHA hamsters are susceptible to a fatal Pichinde virus infection and are unable to limit viremia. Adult hamsters of other strains, such as LVG and LSH, limit viremia and do not develop fatal disease. Both survival and the ability to limit viremia are each controlled by a dominant gene. During studies to delineate the role of cytotoxic cells in Pichinde virus infection, it was observed that MHA hamsters exhibited appreciable levels of cytotoxic activity against syngeneic and allogeneic tumor cells and that this activity increased after Pichinde virus infection. In contrast, LSH hamsters showed lower endogenous cytotoxicity and less enhancement after virus infection. The cytotoxicity was primarily mediated by a cell with characteristics of natural killer (NK) cells. The separation of MHA hamster spleen cells by velocity sedimentation revealed that fractions of the gradient containing maximum NK activity also contained numerous infectious centers, whereas the corresponding fractions from gradients of LSH hamster spleen cells showed less cytotoxicity and contained one-tenth the number of infected cells. These observations suggest that a population of cells, defined by NK activity, may be a target for Pichinde virus replication, and that the activity of these cells is only minimally expressed in the resistant LSH strains.