A cDNA library was constructed from the mRNA of adult worms of Schistomsoma mansoni in the expression vector lambda gt11 and screened with a rabbit antiserum raised against a 60-65-kDa electroeluted adult worm fraction. Two overlapping clones were selected and a partial nucleotide sequence was deduced (1172 bp). The full-length sequence was obtained by the amplification of the 5' end of first strand cDNA using PCR. The overall mRNA size was 1335 nt including a 25 nt 5' non-coding region and a 131 nt untranslated region with the poly(A) tail. The predicted amino acid sequence of 393 aa (45 kDa) has 52% identity with the human Ro/SS-A autoantigen, which is considered to be the human calreticulin. As for the human Ro/SS-A, the protein encoded by the cDNA described here contains a hydrophobic leader sequence and a carboxyl terminal sequence, HDEL consensus signal sequence for retention in the ER. An antiserum raised against the fusion protein of one clone recognized a 58-kDa antigen in homogenates of cercariae and of adult worms. The expression of the protein in the pGEX-2T fusion system allowed us to show the presence of specific antibodies in S. mansoni infected patients' sera and in the sera of patients with systemic lupus erythematosus, reflecting a cross-immunoreactivity between the S. mansoni protein and the human calreticulin autoantigen.