The authors employed serum paper electrophoresis, Immunoelectrophoresis in agar and fractionation on Sephadex G-200 column, to study the serum proteins in 7 patients with South American blastomycosis. Some patients had pulmonary lesions which were associated or not with enlargement of lymph nodes, others presented localizated or generalized enlargement of lymph nodes. In these patients with generalized enlargement of lymph nodes, they found or not lesions of intestinal tract demonstrated through X-ray. They conclude that immunoglobulins G and M are increased in Sout American blastomycosis. The authors suggest that immunoglobulin G is increased whether by chronic granulomatous infection or when this disease progress to fibrosis; it is also related with specific immune response. An increase in immunoglobulin M in the acute phase or recurrence of the disease is demonstrated when there are necrosis or exsudation.