Trypanolytic activity and agglutinins for T. musculi were demonstrated in sera from refractory hosts. The agglutinins in human and bovine serum were specific antibodies. The trypanolytic activity was a result of the ability of the trypanosomes to activate complement in these normal sera. The results suggested that Trypanosoma musculi activates human complement by the alternative pathway. The activity was inhibited by EDTA but not EGTA, and trypanosome lysis occurred in the absence of C2. In addition, conversion of C3 occurred in the presence of EGTA. The trypanolytic activity of bovine serum was similarly inhibited by EDTA but not EGTA. Trypanosome lysis failed to occur in C6 deficient rabbit serum, showing that the late components of complement are required for parasite lysis. Trypanosome lysis by human or bovine serum was inhibited by the addition of mouse serum but not rat serum. These observations suggest that the presence of trypanolytic activity and antibodies to this trypanosome in sera of normal mammals may be responsible for the restricted host range of the trypanosome, and that the absence of these antibodies and the ability of this parasite to evade the trypanolytic activity enables T. musculi to establish infections in the mouse.