SDS-PAGE and N-terminal sequence analysis of hydrolysis products from 3 substrates containing a unique sensitive bond usually recognized by chymosin (rc-casein, a synthetic hexapeptide and a recombinant tripartite protein) revealed that a 45 kDa endoprotease of Myxococcus xanrhus DKlOl cleaved the same characteristic Phe-Met bond with high specificity. Such an enzyme, easy to obtain from culture supematant and to use in acidic conditions, could be a new tool for protein engineering.