Infections due to Taenia solium in humans (taeniasis/cysticercosis) remain a complex health problem, particularly in developing countries. We identified two oncosphere proteins that might protect the porcine intermediate host against cysticercosis and therefore help prevent disease in humans. One of these proteins was further identified by two-dimensional gel electrophoresis and micro-sequencing. The gene encoding this protective protein was also identified, cloned and characterized. The native 31.5 kDa protein Tso31 has four variants at the cDNA level. The longest sequence from which the others seem to derive, encodes a 253 amino acid peptide. The predicted protein has a molecular weight of 25.1 kDa, one putative N-glycosylation site, two fibronectin type III domains, and one C terminal transmembrane domain. The gene structure of the protein consists of four exons and three introns. The finding of one gene and four different cDNAs for Tso31 suggests the existence of a possible mechanism of differential splicing in this parasite. The Tso31 protein is exclusive to T. solium oncospheres with a putative protein structure of an extra-cellular receptor-like protein. The Tso31 protein was expressed as a recombinant protein fused to GST and tested in a vaccine to determine its effectiveness in protecting pigs against cysticercosis. Only two pigs out of eight vaccinated were protected and although the total median number of cyst decreased in vaccinated pigs compared to controls this decrease was not statistically significant (P = 0.09).