Macrophages from the peritoneal cavities of normal mice or mice previously immunized with epimastigotes of Trypanosoma cruzi were incubated with sera from normal or immunized mice and then infected with T. cruzi epimastigotes or trypomastigotes. After 2 hours of incubation the specimens were prepared for scanning electron microscopy. Macrophages from immunized mice were observed to be qualitatively and quantitatively more effective in binding either parasite form than were macrophages from normal mice. Preincubation with specific antibody appeared to enhance parasite binding in all cases, and extracellular destruction of both epimastigote and trypomastigote forms was noted in these preparations. While extensive destruction of epimastigotes was seen with either normal or immune macrophages pre-incubated with antibody, lysis of trypomastigotes was typically seen only in the presence of both immune macrophages and specific antibody.