OmpT is a protease present in the outer membrane of Escherichia coli. The enzyme is tentatively identified as a serine protease and specifically hydrolyses peptide bonds between positively charged amino acid residues. A method has been developed to produce OmpT in large quantities from inclusion bodies. Active enzyme is obtained after refolding, but autoproteolysis was found to be a major drawback of the procedure. The autoproteolysis site was located between Lys217 and Arg218 of OmpT and several mutations were made at this site attempting to prevent autoproteolysis. A strong reduction in self destruction was found for the double mutant G216K/K217G without major losses in overall catalytic activity on model substrates. It was found, however, that autoproteolysis is not fully blocked by the mutations particularly when mutant enzyme is stored at high concentrations e.g. as required for crystallisation studies. Only after additional mutations involving identified active site residues stable protein could be obtained. Interestingly, proteolytic activity has to be reduced by several orders of magnitude before the protein is sufficiently stabilised. The importance of good assay systems will be discussed.