Antigens prepared from cultures of Trypanosoma cruzi were tested by the quantitative complemento fixation test, with a Chagasic reference serum. The titer was computed as the slope of the linear relatioship between antigen (or serum) required for 50% hemolysis and the amount of complement present. When the antigen titer was divided by the serum titer, an index of specific reactivity was obtained. From the values of this index the reprodutibility of antigens was evaluated and also the decay of the capacity of the reativity against the reference serum. It was observed some variations from different vials and some decay of the specific reactivity after 8 months, when kept at 3-6° C. From five antigens prepared by similar technics, two presented comparable reactivity (CDC-10-75 and B.W. 89). The relative discrepancies of specific reactivity were within acceptable limits with the antigens B.W. 89 and B.W. 105, in titrations performed at different time. The highest discrepancy was observed when the antigen 760130 was compared with CDC-10 75, B.W. 89 and B.W. 105. The index of reactivity capacity permitted the evaluation of antigens prepared from, informing on their reproductibility and stability.