The in vitro induction of the cytochrome P1-450-dependent monooxygenases, aryl hydrocarbon hydroxylase (AHH) or ethoxyresorufin O-deethylase (EROD) by 2,3,7,8-TCDD and related toxic halogenated aryl hydrocarbons in rat hepatoma H-4-II E cells has been developed as a short term quantitative bioassay for these toxic chemicals. There was a linear correlation between the -log EC50 (in vitro) AHH induction vs the -log ED50 (in vivo) for body weight loss, thymic atrophy, hepatic AHH and EROD induction in the rat for several polychlorinated biphenyl, dibenzo-p-dioxin and dibenzofuran congeners and mixtures. These data clearly support the utility of the in vitro AHH induction assay as a short term test system for quantitating the 'toxic or 2,3,7,8-TCDD equivalent' in an extract containing toxic halogenated aromatics. The bioassay method is rapid, relatively accurate and much more cost effective than conventional analytical methods such as gas chromatography-mass spectrometry from which it is difficult to determine the levels of 2,3,7,8-TCDD equivalents in specific analytes.