A monoclonal antibody was produced by cell fusion from mice immunised with Trypanosoma cruzi epimastigotes. The antibody was epimastigote specific but not strain specific; the antibody bound to Y, Peru and Tulahuén epimastigotes but did not bind to Y amastigotes or trypomastigotes. The antigen recognised by the monoclonal antibody was a 72 000 molecular weight cell surface glycoprotein which represented only 0.04% of the whole cell protein. Analysis of the glycoprotein purified by antibody affinity chromatography revealed a carbohydrate content of 52% by weight which was composed of glucosamine, mannose, galactose, glucose and three different pentoses: fucose, xylose and ribose. Immunisation with the purified glycoprotein did not protect mice from a lethal infection of T. cruzi.