Objective To study the biochemecal and immunological characterization of the 200 kD schistosomulum surface antigen Method and results A very high molecular weight schistosomulum surface antigen of Mr＞200kD was identified and characterized using monoclonal antibodies. Carbohydrate modification experiments followed by radioimmunobinding assays demonstrated that the epitope recognised by the mAbs on this antigen was carbohydrate in nature, while protein digestion experiments followed by SDS-PAGE indicated that this antigen also contained protein. Immunoprecipitation of 125I-labelled cercarial, schistosomulum, adult worm and miracidial surface antigens followed by gel analysis showed the carbohydrate epitope to be present on 5 cercarial, 2 schistosomulum and 5 miracidial surface molecules, and suggested a possible ecological function involved in adapting the parasite to the aquatic free-living stages of its life cycle and possibly also in protecting the early schistosomula from host immune damage. The 5 cercarial surfacs antigens proved to be associated with the CHR, since all the mAbs which recognised those antigens could induce a strong CHR. A kinetic investigation of the carbohydrate epitope on schistosomula of different ages demonstrated a gradual and possibly irreversible loss during the culture period. The epitope completely disappeared from the surface of adult worms. Conclusion To demonstrate an epitope common to a number of surface molecules of various developmental stages of schistosome and therefore explains the immunological cross - reactivity among different stages at the molecular level.