Specific human IgG antibodies bound to a Trypanosoma cruzi envelope were internalized by antigen receptor-mediated endocytosis. Ferritin conjugated antibodies and horseradish peroxidase (HRP) conjugated IgG were found inside parasite cytoplasmic vesicles. Nonspecific IgG that did not bind to the external membrane was not internalized by the parasite. The ratio of 3H-protein A labeled: specific IgG internalization by parasites in the exponential growth phase (95% epimastigotes) was much smaller than that of parasites in the late stationary growth phase (38% trypomastigotes). Antibodies bound to the latter parasite forms almost disappeared from their outer membranes after 12 hr incubation at 27°C. Results of experiments in which membrane bound antibodies were removed by an excess of pronase showed that only small amounts of radiolabeled IgG were found inside the parasites. The fate of immunoglobulins that vanished from external membrane receptors and did not accumulate inside the cells was explained by experiments in which the supernants of IgG-3H-protein A labeled parasites were precipitated with trichloroacetic acid (TCA). In these, membrane-bound antibodies were taken in and degraded by the parasites as increased amounts of free radiolabel appeared in the supernatants as functions of incubation time and parasite stage.