Taenia solium Linnaeus, 1758 is responsible for taeniasis and cysticercosis, which are 2 serious health problems, particularly in developing countries. The attempt to identify a 22.5kD possible protective oncospheral antigen by 2-dimensional gel-electrophoresis, micro-sequencing, and cDNA library screening produced a protein of 42kD that possesses a conserved domain similar to that of troponin T. Five variants that showed differences at the 5' end were observed at the cDNA level. Hyper-immune rabbit sera developed against recombinant GST fused protein identified the protein exclusively on activated oncospheres. The 42kD protein was tested in an enzyme-linked immunoassay (ELISA) alone and then together with the Tso31 protein for the diagnosis of human cysticercosis. When both antigens were combined, the test was found to be 85% sensitive and 65% specific. The 42kD is a novel T. solium protein that is present exclusively on activated oncospheres of this parasite, with poor diagnostic activity against taeniasis or human cysticercosis.