A PCR strategy using degenerate oligonucleotide primers based upon consensus sequences of the active site of serine proteases yielded a 467 bp fragment from genomic DNA from Schistosoma mansoni cercariae. The sequence presented a continuous open reading frame and the deduced amino acid sequence (156 aa) presented homologies with various serine proteases, in particular the highest percentage identity was observed with a mammalian plasma kallikrein. The expression of this serine protease was studied first at the mRNA level and it was only detected by RT-PCR in cercariae and in adult worms. At the protein level we were able to detect it by Western blotting and by using antigen extracts from metabolically radio-isotope labelled worms. The absence of any positive signal in Northern blot and the detection of the protein suggest that the mRNA has a very short half-life, however the protein may be accumulated in the parasite. The significance of identity with mammalian kallikrein was confirmed by cross-immunoreactivity with a native porcine pancreatic kallikrein. However, no cross-reactivity was observed with S. mansoni elastase, another serine protease. Thus, we suggest that the serine protease described in this paper is a kallikrein-like protease.