The effect of length of treatment of Trypanosoma cruzi with Bayer 2502 (Bay 2502, Lampit, Nifurtimox) on resistance to challenge and parasite isolation was studied in two experiments. In each, 200 mice were divided into groups of 20, infected with T. cruzi, and then treated with Bay 2502 for 1 to 8 consecutive weeks. Sixteen weeks after exposure, 10 mice from each experimental group were challenged with 150,000 trypomastigotes. The remainder were maintaned for 8 more weeks and killed for isolation of parasites. Mice treated with Bay 2502 for up to 8 consecutive weeks were uniformly resistant to challenge, and parasites could not be isolated from groups treatef for over 6 weeks. Results suggest that prolonged treatment with Bay 2502 eliminates T. cruzi from infected hosts and that resistance to challenge is not dependent on the presence of the organisms in mice.