Transmission electron microscopy was used to analyse the process of interaction of Trypanosoma cruzi with resident and activated mouse peritoneal macrophages. Initially, the parasites are located within a membrane-bounded endocytic vacuole. Lysosomes from the host cell fuse and discharge their content into the parasite-containing vacuole, as visualized by localization of horseradish peroxidase and acid phosphatase activity. Acridine orange was used to label secondary lysosomes in order to quantify the process of lysosome-phagosome fusion by fluorescence microscopy. The fusion index was higher for amastigote than for epimastigote and trypomastigote forms. Images were obtained showing that a few hours after ingestion of trypomastigote forms by the macrophages there is progressive disruption of the membrane lining the vacuole, until its complete disappearance.