Mesorhizobium sp. F28 was used in the NHase/amidase enzyme system to convert acetonitrile into acetamide and acetic acid, and the cells grew with the production of acetic acid. The NHase activity of the strain F28 was 78 U mg(-1) dcw, observed in the conversion of 19.5 mM acetonitrile at 0.2 h. As the initial pH value was between 6.5 and 8.3,18.3 mM acetonitrile completely converted into acetamide within 2 h and the accumulation of acetamide subsequently converted into acetic acid and ammonia within 46 h. When 20.3 mM acetamide was added in the medium, the conversion rate of acetonitrile was 80% at 2 h and the conversion rate of the accumulative acetamide was slightly affected. The concentrations of acetic acid and ammonia were respectively 6.01 and 6.68 mM at 46 h. The addition of acetic acid decreased the activities of the NHase and amidase. The conversion rate of acetonitrile was 94% at 9.5 h and traces of acetic acid (0.25 mM) and ammonia (0.29 mM) were produced. The effects of product-inhibition indicated that the appropriate operation of bioreactor would be beneficial for Mesorizobium sp. F28 to degrade acetonitrile continuously. (c) 2008 Elsevier B.V. All rights reserved.