A method for isolation of membrane and flagellar fractions from Trypanosoma cruzi epimastigotes, and descriptions of their ultrastructural characteristics and antigenic activity are presented. Electron-microscopic observation revealed that the membrane fraction resembled vesicles, as occurs frequently when the pressure-depressure method is used for cell disruption. The flagellar fraction revealed flagella and some membranes, both related and unrelated to the flagellar structure. Flagellar and membrane fractions had 5 and 3 precipitin lines respectively against anti-whole homogenate of T. cruzi, in double diffusion tests. One of the precipitin bands common to both fractions gave a reaction of identiy with one precipitin line of the microsomal fractions, that was not present in the mitochondrial, nuclear and cell sap fractions. Data from tests of protective activity activity obtained during a period of 3 years with whole homogenate and F and M fraction are also presented Protective activity against lethal challenge doses of trypomastigotes is strongly associated with the flagellar fraction.