Abstract: A method for producing the derivatives of the invention by preparing a DNA fragment comprising at least the part of the coding sequence of staphylokinase that provides for its biological activity; performing in vitro site-directed mutagenesis on the DNA fragment to replace one or more codons for wild-type amino acids by a codon for another amino acid; cloning the mutated DNA fragment in a suitable vector; transforming or transfecting a suitable host cell with the vector; and culturing the host cell under conditions suitable for expressing the DNA fragment. Preferably the DNA fragment is a 466 bp EcoRI-HINDIII fragment of the plasmid pMEX602SAK, the in vitro site-directed mutagenesis is performed by an oligonucleotide-directed mutagenesis system using the plasmid pMa/c and the repair deficient E. coli strain WK6MutS, and the mutated DNA fragment is cloned in E. coli strain-WK6.