The HSD-I gene codes a human sperm membrane protein (hSMP-1) and has been assigned the accession number U12978. The gene is located on human chromosome 9, region p12-p13. When the 1.7-kb cDNA of HSD-I was digested sequentially with EcoRI, BamHI, and HindIII, a 550-bp cDNA fragment was formed, which codes for the extracellular domain. This fragment was cloned into the asd+ vector pYA3149 to construct pYA3149R. The recombinant plasmid was used to transform an avirulent deltacva, deltacrp, deltaasd vaccine strain of Salmonella typhimurium chi4550. The hSMP-1 component was localized on the surface of the head of mature rat spermatozoa by an immunofluorescence technique using polyclonal anti-hSMP-1 antibodies. Since rat sperm contain hSMP-1, this rodent can be used to assay the immunogenicity of pYA3149R. Female Wistar rats were immunized by oral administration of the recombinant Salmonella. Anti-hSMP-1 antibodies in blood and vaginal washes of immunized animals were determined. Both body fluids contained significant amounts of the antibodies, showing that the recombinant Salmonella is an effective oral immunogen in rats.