GP50, a Taenia solium protein diagnostic for cysticercosis has been cloned, sequenced, and characterized. GP50 is one diagnostic component of the lentil lectin purified glycoprotein (LLGP) antigens that have been used for antibody-based diagnosis of cysticercosis in a Western blot assay for nearly 15 years. GP50 is a glycosylated and GPI-anchored membrane protein. The native protein migrates at 50kDa, but the predicted molecular weight of the mature protein is 28.9. Antigenically active recombinant GP50 has been expressed in a baculovirus expression system. The antigenic activity of both the native and recombinant proteins is dependent upon the correct formation of disulfide bonds. GP50, purified from cysticerci, has two homologs expressed in the adult worm, TSES33 and TSES38. Both are diagnostic for taeniasis. In spite of the amino acid similarities between GP50 and the TSES proteins, each appears to be a stage-specific antigen. A preliminary evaluation of recombinant GP50 in a Western blot assay showed 100% specificity for cysticercosis and 90% sensitivity for cysticercosis positive serum samples reactive with the GP50 component of LLGP.