Blood transfusion is the second most important mechanism of transmission of Chagas' disease, and crystal violet is currently used in blood banks in endemic areas in attempts to eliminate such transmission. A photodynamic action of crystal violet against Trypanosoma cruzi trypomastigotes in blood has been detected. This action was enhanced by addition of sodium ascorbate. Photoirradiation of whole blood containing crystal violet increased the concentration of ascorbyl radical and the generation of superoxide anion. Similar results were observed in incubations containing ascorbate and crystal violet in the absence of blood. Hydrogen peroxide generation was also detected in these incubations, thus confirming redox cycling of crystal violet under aerobic conditions. Since photoirradiation and addition of sodium ascorbate reduces significantly the effective dose and time of contact of crystal violet with T. cruzi-infected blood, a possible practical application of these findings is envisaged.