Trypanosoma musculi infection affected natural killer (NK) activity in mice. In the spleen, an increase of two to three times normal was displayed on days 2 to 4 after inoculation of parasites, followed by rapid decline to a subnormal level of activity that persisted for more than 3 weeks and included the phase of rapid parasite elimination. NK activity increased dramatically in peritoneal exudate and marrow early in infection, but the subsequent decline was more moderate than in the spleen. The subnormal splenic activity was not elevated by treating infected mice with an interferon inducer, polyinosinic acid-polycytidylic acid. Serum interferon levels were elevated early in infection, but by day 4 postinoculation, they had returned to undetectable. Injection of mice with antiserum to murine interferon-beta did not inhibit the early rise in NK activity or alter the course of trypanosome infection; in fact, the antiserum treatment enhanced splenic NK activity in infected mice. The early rise and subsequent decline of NK activity did not correlate with the course of T. musculi infection and subsequent cure. The cause of the dramatic decline in splenic NK activity is under investigation; it could result, for example, from arousal of suppressor cells, inhibition by prostaglandins, or inhibition by trypanosome-derived substances. Thus, NK cells may be prevented from fulfilling their potential of attacking the extracellular trypanosomes by the effects of inhibitory substances.