Abstract: In the present paper, the results of detecting circulating antigen and/or antigen-anti-body complexes by McAb against surface membrane antigen of adult Schistosama japo-nicum were reported. The McAb, coded as 8SE4, was prepared by fusion of SP2/0 myeloma cslls with spleen cells of the BALB/c mice immunized with the saline extract of adult S, japonicum. The 8SE4- directed antigen was proved to be located on the surface of the adult worm. After being purified by a DE52 column, 8SE4 was labelled with HRP and the conjugate (HRP-8SE4) was used in the test. For testing, the serum sample was first incubated with HRP-8SE4, then PEG (mw. 6 000) was added to precipitate the antigen-antibody complex. Upon centrifugation, OPD was added to the precipitate. Results were read by ELISA reader at 492nm. The OD value was found to be proportional to the amount of circulating antigen and/or antigen-antibody complexes. Results from 5 heavily infected (1 500-2 000 cercariae) rabbits showed that the OD values were raised signifi-cantly at the 6th week post infection, being 1.9-4.5 times higher than those before ins faction. The OD values of the 5 rabbits each lightly injected with 10-500 cercariae were also markedly raised 6 weeks post infection and reached the peak at the 8th week, then maintained in high levels until 11th week post infection. The worm burden of the 5 lightly infected rabbits were 4-326. No obvious correlations between OD values and worm loads were observed. The results suggested the existence of surface membrane-related an-ligen and/or antigen-antibody complexes in the circulation of infected rabbits.