We have found that sera from humans with Chagas' disease and Rhesus monkeys infected with Trypanosoma cruzi contain IgM and IgG antibodies, which react with structures in a variety of connective tissues. These antibodies react with laminin but not with various other purified connective tissue components like collagen types I, III, IV, and V, fibronectin, heparan sulfate (BM-1) proteoglycan, or chondronectin. The tissue-reacting antibodies were isolated by absorption to a laminin-Sepharose column. The bound fraction contained all the tissue-reacting antibodies. These antibodies strongly stained trypomastigotes and amastigotes, but weakly stained epimastigotes. These studies show that sera from T. cruzi-infected primates contain antilaminin antibodies, which may be produced by those host in response to a laminin-like molecule present in the parasite.