Trypanosoma cruzi, an obligate intracellular protozoan parasite, chronically infects mammals and causes Chagas' disease in humans. T. cruzi evasion of the mammalian immune response and establishment of chronic infection are poorly understood. During T. cruzi infection, amastigotes and trypomastigotes disseminate in the mammalian host and invade multiple cell types. Parasite surface carbohydrates and mammalian lectins have been implicated in the invasion of mammalian cells. A recent study has demonstrated that the human mannose-binding protein and the macrophage mannose receptor, two mammalian C-type lectins, bind to T. cruzi (S. J. Kahn, M. Wleklinski, A. Aruffo, A. Farr, D. Coder, and M. Kahn, J. Exp. Med. 182:1243-1258,1995). In this report we identify the major surface glycoproteins, including the SA85-1 glycoproteins, as T. cruzi ligands of the mannose-binding protein. Further characterization of the interaction between the mannose-binding protein and T. cruzi demonstrates that (i) the SA85-1 glycoproteins are expressed by amastigotes and trypomastigotes but only amastigotes express the mannose-binding protein ligand, (ii) treatment of amastigotes with alpha-mannosidase inhibits the binding of mannose-binding protein, and (iii) amastigote binding of mannose-binding protein is stable despite the spontaneous shedding of some glycoproteins from its surface. Together, the data indicate that developmentally regulated glycosylation of surface glycoproteins controls the expression of ligands that affect the interactions between T. cruzi and mannose-binding protein. It has been established that the binding of mannose-binding protein to microorganisms facilitates their uptake into phagocytic cells. Preferential opsonization of amastigotes with mannose-binding proteins may account for their clearance from the circulation and may contribute to the parasite's ability to invade different cell types.