Production of clavulanic acid (CA) by Streptomyces clavuligerus in a shake-flask culture increased from 92 to 180 mg l(-1) with an increased O-2 transfer efficiency (0.039 --> 0.058 s(-1)), which maintained the redox potential values above -250 mV. Compared with traditional measures, such as dissolved O-2 concentration and respiratory activity, the redox potential can easily be determined and correlates closely with CA production. It can therefore be used to monitor microbial activities during biosyntheses of secondary metabolites.