The induction of mixed function oxygenase (MFO) enzymes in fish exposed to pulping effluents is well documented but the responsible compounds are unidentified. Liver ethoxyresonifin-O-deethylase (EROD) activity was determined in rainbow trout after exposures to fractions generated from the effluent of a modernized bleached kraft pulp and paper mill. Exposures to whole and filtered (<1 μm) effluent, resuspended solids, and two fractions generated from nanofiltration were conducted for primary effluent, effluent after secondary treatment (aerated lagoon), and effluent collected during a scheduled pulping shutdown. For each fraction exposed, effluent constituents were detected using GC with ECD, and FID as well as GC-MS. Exposure concentrations were compared with EROD activities to evaluate their potential to cause induction. Resin acids, fatty acids, bacterial fatty acids, terpenes, chlorophenolics, aliphatic alkanes, plant sterols, and chlorinated dimethylsulfones for which authentic standards were available were eliminated as potential EROD inducers. Several chlorophenolics, including tetrachloroguaiacol, exhibited a correlation with observed induction patterns but subsequent exposures to pure tetrachloroguaiacol failed to cause MFO induction. The correlations exhibited by these compounds may indicate the potential source of the chemicals responsible for MFO induction.