Cell-mediated immunity is a subject of intense interest in parasitology. Experiments with animals and observations on humans suggest that cell-mediated immunity is involved in protozoal infections (1). The inhibition of leukocyte migration has proved a suitable method for the in vitro study of cellular immunology. Søborg and Bendixen (2, 3) have successfully used this test with human peripheral leukocytes; Rosenberg and David (4) have shown it applicable to soluble antigens. In the present work we have attempted to obtain an in vitro test of the immune mechanisms in American trypanosomiasis. We observed the in vitro migration of peripheral leukocytes from patients with positive serology for Trypanosoma cruzi infection against soluble antigen. Materials and Methods. Blood. Two samples from each of 30 human adults were drawn: one, with heparin (Abbott 5000 U/ml with benzyl alcohol preservative) for the migration test, the other for the serologic tests.