Scale-up of production of an alkaline protease, previously characterised from a new isolate of Bacillus subtilis for use as a bating enzyme in leather processing, is described. Before large-scale commercial production of the protease is possible, characteristics of the growth of the bacterium and enzyme production in fermenters must be defined. In 2 dm(3) fermenters an optimal specific activity of 296 x 10(3) U g(-1) cell dry weight was obtained after 60 h with the dissolved oxygen tension (DOT) kept above 10% and pH left uncontrolled. Culture pH was 6 on inoculation, falling to 5.3 after 12 h before rising steadily to similar to 8 at the end of fermentation. DOT was maintained above 10% by agitation in the range 300 to 500 rpm. The same criteria were adopted for scale-up to 20 dm(3) but the increase in activity occurred 24 h later.