The failure of clonorchis ova to elicit antibody in infections is not due simply to the rarity with which ova enter the tissues. The ovum does not provoke granuloma formation as some other ova do when it is injected experimentally into the subcutaneous tissue of mice in the absence of bile. Granulomatous appearances in occasional human lesions in which ova are found in the tissues outside the biliary tree are due to the presence of bile. The circum-oval precipitin test is consistently negative and the shell of the clonorchis ovum is impermeable to the antigens in the embryo. It is concluded that clonorchis ova play no important role in stimulating humoral and cellular immunity in the definitive host.