A direct immunofluorescent antibody test with an anti-Trypanosoma cruzi F(ab')2 conjugate was used to demonstrate antigens of T. cruzi on the membrane surface of intact live or fixed macrophages and L929 mouse fibroblasts infected with the organism. Antigens were demonstrated in 5 to 50% of infected cells, and their presence was not directly related to the number of intracellular organisms. Cells with as few as four intracellular amastigotes had demonstrable surface antigens, whereas some cells with as many as twelve or more organisms did not. Capping of antigen-antibody complexes was noted to begin a few minutes after the addition of the anti-T cruzi F(ab')2 conjugate; by 30 min, most of the parasitized cells had eliminated the complexes, and no surface antigen of parasitic nature could be demonstrated. Although capping may have caused a negative result in a previously positive cell, other mechanisms may be involved, because antigens were not demonstrated in some heavily parasitized cells examined immediately after completion of the test. Treatment of the infected cells with trypsin or chymotrypsin resulted in the absence of demonstrable parasite antigens on the cell membrane surface. However, the antigens were again demonstrated 12 hr after the enzymes were removed. The reappearance of parasite antigens on the surface of infected cells was prevented by treatment of the monolayers with puromycin or tunicamycin. A T cell-enriched population of spleen lymphocytes from mice chronically infected with T. cruzi recognized the membrane-bound antigens and proceeded to destroy the host cell and the intracellular organisms. In this process, noninfected cells were also destroyed, possibly because they were coated with antigens released from intact infected cells or from infected cells that had been lysed by the action of the sensitized lymphocytes or their products.