To produce a large amount of recombinant proteins in Escherichia coli, we constructed a unique cis-acting expression system using a plant virus protease. This new expression system could directly produce recombinant proteins, that had a biologically active form. A gene of nuclear inclusion-a (NIa), which had a specific amino acid sequence, was fused with a foreign protein gene at the same protein reading frame. One of the NIa-specific cleavage amino acid sequences, Gln-Ala, was also contained at the protein-protein junction. In the case of human interleukin-11 (hIL-11), a 23-kDa specific signal band was obtained from recombinant bacteria. N-terminal sequencing of the 23-kDa protein showed that NIa specifically cleaved the fusion protein at Gln-Ala, producing Ala-hIL-11. Furthermore, we could produce the mature rhIL-11 by extending the culture time. This 23-kDa protein had the same biological activity as hIL-11 in a mouse plasmacytoma, T1165. Combined with fermentation control, we produced mature rhIL-11 in E. coli.