In order to explore the functions of f ish cystatin and the potential values in the f ish disease prevention and cure, as well as seafood process industry, polymerase chain reaction ( PCR) technique was used to modify the cDNA of Chinese sturgeon ( Acipenser sinensis ) cystatin mature peptide, The cDNA was subcloned into expression vector pBV220, then a cystat in recombinant expression bacteria was constructed. After temperature inducement and SDS..PAGE analysis, the recombinant expression bacteria produced a special protein about 12. 4 kD in molecular weight. The proportion of recombinant protein in total bacterial protein was about 25%. The purity of the recombinant cystatin, which was washed, dissolved, dialysed and renatured, was about 85%. The inhibitory activity of recombinant cystatin could be measured by inhibiting the protease activity of papain. The inhibitory activity of recombinant cystatin was 48U....g- 1.