Rats were infected orally with 50 or 100 metacercariae of Clonorchis sinensis. Anti-Clonorchis IgE antibody, determined by passive cutaneous anaphylaxis, in the serum of infected rats appeared 30–40 days after infection and persisted for at least 120 days. Total amount of IgE fixed on the mast cells, detected by reverse passive cutaneous anaphylaxis with anti-rat IgE, was significantly more in the infected rats than in uninfected rats. Worm burden at 290 days post-infection did not correlate to anti-Clonorchis IgE antibody titers, reverse passive cutaneous anaphylaxis titers, or anti-Clonorchis anaphylactic antibody on the mast cells, determined by active cutaneous anaphylaxis. Active cutaneous anaphylaxis titers interrelated to reverse passive cutaneous anaphylaxis titers, suggesting a correlation between the amount of anti-Clonorchis anaphylactic antibodies and total IgE on the mast cells. Sensitivity of passive cutaneous anaphylaxis with anti-BSA IgE antibody was significantly suppressed in Clonorchis infected rats. Moreover, anti-BSA IgE titers in the infected rats were inversely related to reverse passive cutaneous anaphylaxis titers, indicating that increase of IgE antibody on the mast cells by Clonorchis infection interfered with sensitization of anti-BSA IgE antibody.