Methodology was developed in order to rapidly and cost-efficiently screen whole water samples without extraction for the presence of dioxin-like compounds using a cell bioassay approach. Presence of dioxin-like compounds was indicated by the induction in the rainbow trout (Oncorhynchus mykiss) liver cell line, RTL-W1, of cytochrome CYP1A, which was measured as 7-ethoxyresorufin-O-deethylase (EROD) activity. Two simple culture media, L-15/ex and Earle's-G, prepared in tissue culture water and supplemented with 5% serum, proved suitable for supporting RTL-W1 cell viability and induction of EROD activity by the model inducers, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene (BaP). Preparation of the same simplified media using whole surface and ground water instead of tissue culture water again allowed EROD induction by spiked TCDD and BaP to be detected but higher concentrations of inducers were necessary. Despite this reduced sensitivity, RTL-W1 cells responded to 4 out of 40 ground water samples from a former oil and lignite processing site with significant EROD induction. In the future, the value of the bioassay is as an inexpensive means of quickly screening ground and surface water samples to identify high contaminant levels particularly at industrial sites, where detailed site-investigations and long-term monitoring programs are required.