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An immunoglobulin A monoclonal antibody (MAb5C3) was developed against a 15-kDa surface glycoprotein (GP15) of Cryptosporidium parvum sporozoites. Indirect immunofluorescence and colloidal gold immunoelectron microscopy revealed that the antibody reacted with both the sporozoite and merozoite surface plasma membranes. On Western immunoblots, MAb5C3 binding was found to be strongly inhibited when 200 mM N-acetylglucosamine was used as a competing sugar. N-Acetylgalactosamine inhibited binding of the antibody only slightly, whereas glucose, mannose, and galactose failed to inhibit binding. MAb5C3 was found to recognize a similar 15-kDa epitope associated with a Cryptosporidium sp. isolated from guinea pigs. However, MAb5C3 failed to react with any proteins or glycoproteins associated with C. baileyi from chickens, Cryptosporidium sp. (= bovine C. muris) from cattle, C. serpentis from a rat snake, bradyzoites of Besnoitia darlingi from an opossum, sporozoite/oocyst extracts of Caryospora bigenetica from an eastern diamondback rattlesnake, sporozoites of Eimeria nieschulzi and E. papillata from rats and mice, or tachyzoites of Toxoplasma gondii (RH strain). When hybridoma supernatants containing MAb5C3 were administered orally to suckling mice experimentally infected with C. parvum, a 75% reduction in developmental stages was seen histologically at 72 h postinfection and a 67.5% reduction in mean oocyst output was found at 6 days postinfection.