Sm15 is a major Schistosoma mansoni 15 kDa tegumental antigen, resulting from the proteolytic processing of a larger precursor. The amino terminus of Sm15 was identified by direct amino acid sequencing, and the antigen was tentatively mapped to the segment spanning amino acids 362-497 of the precursor. This will allow subsequent studies to elucidate the possible immunological role of proteolytic processing in schistosomiasis.