Reactivity of differentTrypanosoma cruzidevelopmental forms with purified Chagasic anti‐α‐galactosyl antibodies (anti‐Gal) was studied using epimastigotes from axenic cultures, trypomastigotes and amastigotes from infected Vero cell cultures, and an immunogold labeling method as observed by electron microscopy. Epimastigotes were poorly labeled, whereas extracellular trypomastigotes and amastigotes bound heterogeneously to the antibody with many cells being intensely labeled at the cell surface, including the membrane lining the cell body, the flagellum and the flagellar pocket. Parasites with poor labeling at the cell surface generally had several gold particles within the cell, mostly in cytoplasmic vacuoles. The Golgi complex of trypomastigotes was strongly labeled. Intracellular parasites were labeled at the parasite cell surface or within vacuolar structures. The expression inT. cruzi‐infected Vero cells of α‐galactosyl antigenic structures acquired from the parasite was shown by moderate labeling with Chagasic anti‐Gal of the membrane lining parasite‐free outward cell projections. The reactivity with purified anti‐Gal from healthy individuals at the same concentrations of Chagasic anti‐Gal was poor, with gold particles appearing in the nucleus and cytoplasm but not at the cell surface. It paralleled the labeling withBandeireae simplicifoliaIB‐4 lectin. The results provide a basis for autoimmune reactions involving anti‐Gal from ch