A method of antigen preparation and isolation of Schistosoma mansoni cercarial antigens is described. At least nine potentially purifiable antigenic substances from cercariae were detected. Three immunogenic components were purified and analyzed for various immunochemical properties and chemical staining reactions. At least two different antigens of cercariae induce antibody formation in chronically infected human beings. The purified antigen I that was isolated by column chromatography reacted specifically with antibody from patients infected with Schistosoma mansoni. Whether or not one of these antigens can be utilized to induce protective immunity against natural infection is yet to be determined.