The toxicity of C.I. Reactive Black 5 and three Procion dyes, as found in textile effluents, was determined using the
bioluminescent bacterium Vibrio fischeri . Hydrolysed Reactive Black had a slightly greater toxicity than the parent
form (EC50 11.49/3.68 and 27.59/4.01 mg l1, respectively). A baffled bioreactor with anaerobic and aerobic
compartments was used to decolourise hydrolysed Reactive Black 5 in a synthetic effluent. Decolourisation of
hydrolysed Reactive Black resulted in an increased toxicity (EC50 0.29/0.03 mg l1). Toxicity was not detectable when
decolourised Reactive Black 5 was metabolised under aerobic conditions. No genotoxicity was detected after the
decolourisation of either the parent or the hydrolysed reactive dyes, either in vitro or in the bioreactor. The toxicity and
genotoxicity of decolourised C.I. Acid Orange 7 was due to the production of 1-amino-2-naphthol (EC50 0.19/0.03 mg