The sera of 20 patients with paracoccidioi domycosis (South American blastomycosis) were studied using a column of Sephadex G-200 for immunoglobulin separation and agarose gel counterimmunoelectrophoresis for the determination of specific anti-fungus antibodies. Sonicated yeast phase Paracoccidioides cultures were used as antigen. The sera that showed 1, 2, 3 or 4 precipitation bands were separated by column chromatography and the fractions obtained were concentrated and examined by counterimmunoelectrophoresis with the same antigen. Only the fractions with appropriate levels of IgG immunoglobulins reproduced the results obtained with the corresponding whole sera, whereas fractions with normal IgA and IgM levels and with little or no IgG produced no precipitation bands. It is concluded that the precipitating antibodies with specificity for the Paracoccidioides are of the IgG type.