An examination of sera from a range of animals refractory to Trypanosoma lewisi showed that some contain trypanolytic activity and agglutinins for this trypanosome. Trypanolytic activity was demonstrated in bovine, sheep, and rabbit serum. These sera were shown to also contain agglutinins. The trypanolytic activity in all three sera was abolished by heat inactivation at 56 degrees C for 30 min. Studies with bovine serum showed that the lytic activity was totally inhibited by the addition of EDTA but was only partially inhibited by ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid-Mg2+, suggesting the involvement of both classical and alternate pathways of complement activation. Serum from the natural host, the rat but not mouse serum, was capable of inhibiting the trypanolytic activity of bovine serum. The agglutinins in bovine serum were specific, since they could be adsorbed out with T. lewisi but not T. musculi. They fractionated on gel filtration in the position expected for 19S antibodies. By using opsonization measuring techniques, it was found that mouse serum lacked opsonins to T. lewisi, although opsonic activity was detectable in human serum. The absence of agglutinins, trypanolysins, and opsonins in mouse serum suggests that other factors prevent the infection of mice by T. lewisi. From these findings, it would appear that the inability of T. lewisi to infect a range of mammals, with the exception of mice, could be due to the presence of antibodies, the ability to activate complement through the alternative pathway, or both. The absence of these recognition factors and the ability to evade the effects of alternative pathway activation may be important factors in the ability of this parasite to establish infections in rats.