This bioassay utilises cultured H-4-II-E rat hepatoma cells to assess the aryl hydrocarbon hydroxylase (AHH) inducing potencies of planar aromatic hydrocarbons and/or contaminated environmental samples. The response of the cells to pure test chemicals or extracts of mixtures is compared with their response to the standard 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).