Excretory/secretory (ES) antigen of Toxocara canis larvae was isolated from the culture medium of second-stage larvae by gel filtration. The antigen induced production of not only IgM or IgG but also IgE antibody. The antigen, when treated with guanidine hydrochloride and 2-mercaptoethanol, lost its allergenic activity but retained an ability to induce IgM or IgG antibody. Purified antigen was of glycoprotein nature and had a molecular weight of 35,000. The whole antigen showed a cross-reaction with the serum from Ascaris suum-infected mice. The antigen did not contain phosphorylcholine as a structural component.