12 small-scale bubble columns, each and every column with a total volume of 500 mi, individually controlled distribution of sterile and humidified air and a pH-probe, were operated in an incubation chamber with temperature control. Intermittent feeding of substrate or inducer, as well as of base for parallel pH-control was achieved by connecting a precision syringe pump to a valve distributor with one 2/2-way miniature valve for each bubble column. This new parallel bio-reactor technique was applied for optimization of the feed profile of the chemical inducer isopropyl-beta-D-thiogalactoside (IPTG) to improve the expression of a foreign protein (guanosin-5＇-diphosphate-alpha-D-mannose-pyrophosphorylase, GDP-manPP) under the control of the lac promoter in Escherichia coli, A mean cell density of 8 g l(-1) of recombinant E. coli was achieved in parallel fed-batch fermentations within 9.5 h due to a sufficient oxygen transfer (k(L)a of up to 0.2 s(-1)) and parallel pH-control. The GDP-manPP activity was improved by more than 100% compared to the standard IPTG pulse within 3 sets of parallel fed-batch fermentations. A Genetic Algorithm was used for optimization of the IPTG feed profile. Scale-up of the optimized fed-batch process into the stirred tank reactor (scale-up factor of 20) was possible, if all reaction conditions were copied exactly with respect to the reactor volume. A final GDP-manPP activity of 715 U l(-1) was measured 3 h after IPTG induction was finished.