A lambdaZAP II cDNA library was constructed from mRNA in immature seeds of the grass Job's tears. A cDNA clone for a cysteine proteinase inhibitor, cystatin, was isolated from the library. The cDNA clone spanned 757 base pairs and encoded 135 amino acid residues. The deduced amino acid sequence was similar to that of cystatins from the gramineous plants rice, sorghum, and corn. The central Gln-Val-Val-Ala-Gly sequence thought to be one of the binding sites of cystatins was found. A remarkable characteristic of the peptide sequence of Job's-tears cystatin was the putative signal peptide that has been found in sorghum and corn but not in rice. The cystatin cDNA was expressed in Escherichia coli as a His-tagged recombinant protein. The purified recombinant protein inhibited papain.