To improve the sensitivity and specificity of the indirect hemagglutination test (IHA) for the serodiagnosis of Chagas' disease some previously described procedures (sonication, destil-led water, NaOH, TRITON x 100, freezing and thawing) were tested to determine the most useful antigenic extract. Sera of 30 individuals with chronic Chagas' disease, demonstrated by immunofluorescence test, and sera of 30 non •chagasic individuals were assayed. The immunofluorescence test (IFT) was the reference test to evaluate the co-positivity index and co-negativity index. Co-positivity results were better when the NaOH antigen was used. All antigens gave maximum values for co-negativity in relation to IFT. Titers presented by sera from cha-gasics tested by IHA with NaOH antigen were higher than those of the other four antigens. Moreover, reliability of IHA with NaOH antigen was indicated by the sensitivity, specificity and reproducibility observed with five different reagent batches prepared with antigens extracted on different days.