Abstract: A recombinant protein for the diagnosis and immunization of humans and animals against gastrointestinal disorders of Cryptosporidium parvum (C. parvum) infection contains the essential features of the native antigen and may be used in place of the native antigen. A recombinant protein can be quickly and inexpensively reproduced. A DNA sequence encoding a 17-kDa protein for a surface antigen of C. parvum is also disclosed. The DNA sequence may be inserted into recombinant DNA molecules such as cloning vectors or expression vectors for the transformation of cells and the production of the protein. Test kits for detecting contamination of food or water are also provided. Also disclosed are a detergent extraction method for partial purification of the 17-kDa and 27-kDa surface antigens and ELISA assays employing these proteins for detecting C. parvum antibodies in biological samples.