An assay for the evaluation of the antigens reactivity, based in the determination of free complement by the time required for 50% hemolysis, couid be applied for detection of the maximally reactive dose of antigens, in reactions with the reference Chagasic serum. When the logarithms of the time required for 50% hemoiysis are piotted against the time of incubation, the points describe a reggression Une, whose parameters characterized the combining capacity of the immune-complexes, in reactions with complement. The kinectic method is recommended for the preiiminary evaluation of antigens, for its precision and reliability.