Chagas's disease is a chronic, debilitating, incurable disease affecting at least 12 million people in Central and South America (1, 2). TTypanosoma cruzi, the etiological agent, exists in three morphologically distinct forms related to different environments where it lives (1, 3): the amastigotes, which represent the multiplying stage found within the cells of mammalian hosts; the trypomastigotes, which are present in the bloodstream and in infected tissues of mammalian hosts, in the lumen of the rectum of the transmitting triatomid insect, and in culture; and the third form, epimastigotes, which are found intracellularly in mammalian hosts as a multiplying form in the vector's digestive tract, and in culture. Although bloodstream, insect, and culture trypomastigotes are highly infective, culture epimastigotes have very little, if any, capacity to initiate infection in the mammalian host (3). Although the morphologically defined forms of T. cruzi perform different functions in the life cycle of the parasite, the molecular mechanisms underlying the distinct behaviors remain unknown.The majority of exposed plasma membrane proteins are glycoproteins (4, 5), and there is increasing evidence that glycoproteins are of major importance in the maturation and regulation of functions displayed by cells such as those of the immune system (6, 7). On the other hand, much of the work concerning structure and function of cell surface carbohydrates is derived from studies with lectins, a class of sugarbinding proteins of nonimmune origin (8-10).In our study a systematic analysis of cell surface glyeoproteins and/or of glycolipids of T. cruzi developmental stages was undertaken, using ~30 highly purified, glycosidase-free lectins of various carbohydrate specificities as probes. The results showed that some lectins display unique selectivity in interacting with trypanosome forms as determined by agglutination and binding assays. It also showed that trypomastigotes obtained from blood of infected mice and from culture differed in their surface carbohydrates. Moreover, sialic acid residues detected and quantitated in epimasti-