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The toxicity of 17 selected process e uents, 11 reactive dyestu s and 8 auxiliaries from a textile dyeing and ®nishing mill in Ayazaga, Istanbul, Turkey was evaluated by bioluminescence test using bacteria Vibrio ®scheri in LUMIStox 300. The EC 20 and EC 50 for auxiliaries, the EC 20 for dyestu s were determined. For selected process e uents GLvalues, the dilution level at which a wastewater sample causes less than 20% inhibition, were examined. Our results demonstrate that the toxicity assessment with luminescent bacteria is e ective and of practical use for chemicals applied in textile ®nishing industry with the limitation of the deep dark-colored dye bath samples and for the related e uents. Inhibition e ects of numerous dyestu s as well as auxiliaries to luminescent bacteria di ered considerably with a range 5±600 mg l À1 for EC 20 and 9±6930 mg l À1 for EC 50 , respectively. Among 17 e uents, 1 sample exhibited high toxicity (GL 100), 7 showed moderate toxicity (GL 12±32), and 9 had a GL-value <10 indicating a low or no toxicity. Ó