Nine nitroaromatic hydrocarbons (NAHs) induction of EROD activity was investigated by an in vitro assay based on carp (Cyprinus carpio) hepatic microsome system. All nine NAHs tested were found to induce the activity of EROD. There was dose-effect relationship between EROD activity and concentrations of NAHs. It was found that EROD activity was different, when different groups substituted at the same position of benzene ring, or the same group was at different position of benzene ring.