In order to optimize the production of recombinant potato carboxypeptidase inhibitor (rePCI), a protein with 39 amino acid residues and three disulphide bridges, by Escherichia coli MC1061[pIMAM3], the effects of various parameters were investigated. Production of rePCI in M9CAS medium was optimal at 37 degrees C and using low concentrations of glycerol as a carbon source. Increasing concentrations of glycerol caused a decrease in the production of rePCI, which was almost totally inhibited above 1% glycerol. Relatively high concentrations of oligoelements in the culture medium also inhibited the production of rePCI. We obtained a 100-fold increase in the production of rePCI, from 2 to 200 mg/l, when growing bacteria in a fed-batch aerobic culture using a 2-1 fermenter. RePCI was found exclusively in the supernatant, although the genetic construction was designed for it to be released into the periplasmic space. Large quantities of rePCI could be easily purified from the supernatants of these cultures. Our conditions of fed-batch, aerobic fermentation could be used for overproduction to high levels of other recombinant proteins.