Abstract: In this study, naturally occurring anti-idiotypic antibodies in sera from patients with schistosomiasis japonica were demonstrated by using McAb (5B5H)-ELISA. McAb 5B5H against Schistosoma japonicum soluble egg antigen (SEA) was purified by precipitation With 50%, 40% and 33% saturated (NH4)2SO4 and coupled to CNBr-activated Sepharose 4B (Pharmacia). 5B5H column was loaded with sera from two patients with schistosomiasis japonica and then the bound material was eluated with 3M KCNS. The resulting eluate was further characterized with McAb-ELISA and complement fixation test in order to minimize the possible interference of circulating antigen-antibody complex. The results showed that the eluate contained mainly anti-Id antibodies. In the competitive 5B5H-ELISA tests, SEA markedly inhibited the interaction of 5B5H (Id) and anti-Id anti-bodies, suggesting that the relevant idiotype is within or adjacent closely to antigen binding site of anti-SEA molecule. The anti-Id antibodies were detected using 5B5H-ELISA in patients with schistosomiasis japonica from A and B areas, positive rate of 35.1% (13/37) and 60% (18/30) was obtained respectively. No cross or false reactions were found with sera from patients with fasciolopsiasis buski (0/15), clonorchiasis sinensis (0/15) and normal group of students (0/54) with the exception of one out of 20 patients with paragonimiasis westermani.