Abstract An immunoglobulin reacting with anti-human γE antibodies was detected in the sera of monkeys immunized with Ascaris antigen or infested with parasitic mites. The monkey protein gave a precipitin band with antiserum specific for the Fc portion of human γE, but failed to react with any of the antisera specific for the other human immunoglobulin classes. The human γE precipitin band showed a spur over the precipitin band formed between the monkey protein and anti-human γE. The monkey protein reacted with anti-light chains, indicating that it is an immunoglobulin. The physicochemical properties of the protein, i.e., electrophoretic mobility, sedimentation coefficient and chromatographic characteristics, were similar to those of human γE. It was also found that monkey homocytotropic antibodies against Ascaris antigen resided in the protein. These findings indicate that the monkey protein represents a distinct immunoglobulin class, corresponding to human γE. γE-forming cells were found in the lungs and regional lymph nodes of the animals infested with mites.