The sequential change of class-specific antibodies to antigens of the Schistosoma japonicum egg and adult worm in mice after treatment with a single oral dose of praziquantel (PZQ) at 100 mg/kg was studied using ELISA in regard to the resulting release of egg antigen. In 3 out of 4 mice treated with PZQ, only IgM antibody to egg antigen was drastically declined from 30 min to 6 hr after treatment, and then gradually rose to the initial level after 2 to 3 days. After adding 100 micrograms/ml PZQ to the serum from infected mouse in vitro, no reduction of antibody activity was observed. In 2 infected mice intravenously injected with 1.5 mg of egg antigen only anti-egg IgM was remarkably reduced shortly after injection and fatal anaphylactic reactions occurred after 30 or 60 min. By the inhibition ELISA, the amount of circulating immune complex (CIC) in serum was shown to be correlated with increase in amount of egg antigen and with decrease in titers of anti-egg IgM. The results suggested that, after treatment with PZQ, miracidia within eggs in host tissues hatched and that there was increased release of egg antigen which was bound by IgM to produce CIC. This resulted in transiently lower levels of anti-egg IgM antibody. The antigen release may also cause anaphylactic reactions such as abdominal pain and vomiting reported in many post-treatment patients.