The role of antibodies in the previously demonstrated harmful effect of Trypanosoma cruzi-infected mothers on progeny infection was studied by injecting either serum from chronically infected animals or purified T. cruzi-specific antibodies into uninfected mice during gestation and lactation periods. It was verified that injected antibodies were transferred to offspring. Pregnant or lactating animals exhibited lower circulating antibody levels than nonpregnant or pregnant but nonlactating mice, respectively, suggesting that such antibody transfer occurred in both fetuses and suckling offspring. When infected two months after birth, offspring of mice treated with chronic serum or purified antibodies displayed significantly higher parasitemia than offspring from mothers receiving control serum or immunoglobulins unrelated to T. cruzi. These results indicate that soluble factors contained in sera of infected mice, and particularly antibodies, when transferred from mothers to their young, are able to worsen T. cruzi acquired infection in the offspring.