Holzman rats and C3H/Anf mice were infected with the Tulahuén strain of Trypanosoma cruzi. Infected rats had a 16% cumulative mortality during a 49-day observation period. The parasitemias increased to a peak on the 21st day, and then decreased abruptly. In the inoculated mice, maximum numbers of parasites were detected after 15 days and an 87% cumulative mortality was observed during the infection. Three serological techniques using antigens of the metacyclic stages of T. cruzi were employed to compare the levels of humoral antibodies during the early phase of infection. These included: the direct agglutination test (DAT), the indirect immunofluorescent antibody test (IFAT), the enzyme-linked immunosorbent assay (ELISA) which detected IgG (ELISA-IgG) and the ELISA which detected IgM (ELISA-IgM). Antibody titers were first found in antisera from rats after 7 days by the DAT and the ELISA-IgM and after 14 days by the IFAT and the ELISA-IgG. Peak titers were measured on day 21 by the ELISA-IgM. The DAT, the IFAT, and the ELISA-IgG titers increased through 49 days. Antisera collected from mice first reacted with T. cruzi antigens on day 10 in the DAT, the ELISA-IgG, and the ELISA-IgM, and on day 15 in the IFAT. Peak titers were recorded on day 20 with the DAT and the ELISA-IgM. The IFAT and the ELISA-IgG titers continued to rise through 30 days. Alterations in production of antibodies during infections may in part reflect responses to different parasite antigens. The variations in titers of the antisera from infected rodents indicated that metacyclic trypomastigotes share antigens with other stages of the parasite. These metacyclic stage antigens showed a potential for use as serodiagnostic reagents.