Macrophages [from mice] with lysosomes previously labeled with acridine orange or horseradish peroxidase [HRP] were simultaneously incubated in the presence of epimastigotes of T. cruzi and tachyzoites of T. gondii. Light microscopic observations indicate that although a large number of macrophages were infected by the parasites very few contained both parasites. Fusion of the macrophage''s lysosomes with vacuoles containing T. cruzi were seen both by fluorescence (acridine orange) and electron (HRP) microscopy. No fusion of lysosomes was seen with vacuoles containing T. gondii. These results were observed in macrophages infected by only 1 or by the 2 parasites simultaneously.