CSL is a approximately 1300-kDa Ag, expressed by merozoites. CSL is characterized as a soluble glycoprotein exoantigen released by infectious sporozoites. This protein was determined to be the molecular species mechanistically involved in the CSP-like reaction by its identification in SDS-PAGE gels and Western blots of purified membranous precipitates. CSL might has a functional role in sporozoite infectivity and is a candidate molecular target for passive or active immunization against cryptosporidiosis.