Abstract: A novel, generally applicable method for producing correctly folded proteins from a mixture of misfolded proteins, e.g. bacterial inclusion-body aggregates. A major new aspect of the method is that over-all efficiency is achieved by subjecting proteins to a time-sequence of multiple denaturation-renaturation cycles, resulting in gradual accumulation of the correctly folded protein. The method has proven efficient for a variety of recombinant proteins. Also provided are novel encrypted recognition sites for bovine coagulation factor X.sub.a. The encrypted recognition sites described may be activated in vitro by controlled oxidation or by reversible derivatization of cysteine residues and thereby generate new cleavage sites for factor X.sub.a. Two new recombinant serine protease exhibiting narrow substrate specificity for factor X.sub.a recognition sites are also provided. They may replace natural coagulation factor X.sub.a for cleavage of chimeric proteins.