An intradermal reaction is induced by the parasitization with f ilariae. The antibody against these parasites can be detected by the hemagglutination (HA) test using erythrocytes treated with tannic acid. The present experiments were carried out in order to clarify the relationship between the intradermal reaction and the HA test in canine filariasis and Bancroft's filariasis. As a result, no parallel relationship was observed between the intensity of the intradermal reaction and the titer as determined by the HA test. It was presumed that the relationship among the appearance of microf ilariae, the intradermal reaction, and the HA test might be influenced by the differences in the antibody system on the part of the host concerned.