A nitrile hydratase (NHase) gene from Aurantimonas manganoxydans, cloned and expressed in Escherichia coli, gave an enzyme that efficiently hydrated 3-cyanopyridine to nicotinamide with high thermal stability. We have now found that adding Co2+ at 0.1 mM to LB medium was essential for production of an active enzyme. However, a parts per thousand yen0.3 mM Co2+ inhibited the growth of host cells in LB medium and decreased the production of the recombinant NHase. Furthermore, beta-mercaptoethanol promoted regeneration of the Co2+-defective apoenzyme in vitro possibly by breaking a key disulfide bond thereby promoting the incorporation of Co2+ into the apoenzyme.