We investigated the optimum conditions for the formation of nitrile hydratase (NHase), which acts on indole-3-acetonitrile, in Agrobacterium tumefaciens, Good inducers for enzyme formation have been found to be roughly classified into three representative types of amides such as pivalamide, crotonamide and epsilon-caprolactam. When the strain was cultivated in the optimum culture medium containing epsilon-caprolactam as an inducer, in particular, the specific activity of NHase in the culture was 13 000 times higher than that without addition of amides, nitriles or acids. In this case, NHase formed accounted for 12% of the total cellular soluble protein. The purified NHase did not act on epsilon-caprolactam, and epsilon-caprolactam was not degraded during the cultivation by the strain, suggesting that epsilon-caprolactam seems to keep driving the NHase induction mechanism.