This study provides a first attempt from a toxicological perspective to put forward, in significant terms and explanations, decolorization of reactive dyes. The toxicity series of three dyes used is Reactive acid yellow (RAY) > Reactive black B (BB) > Reactive red 22 (RR22). The less toxic dye (e.g. RR 22) is more readily biodecolorized. The oxygen-repression characteristics of decolorization of RR 22 are due to the obligatory requirement of oxygen-sensitive azoreductase expression to proceed decolorization. Therefore, microbial decolorization of RR 22 is termed non-growth-associated. Biotoxicity of BB is greater than RR 22, as approximate to 50% of initial cell concentration was lost after 1-day static decolorization. Relatively high toxicity to P. luteola was due to two azo-bonds present on BB for decolorization. In addition, intermediary products of BB may be more toxic or chemically stable than the parent compound BB. A longer persistence of intermediary metabolites may also augment toxicity to bacterial populations. Biosorption accounts for pseudo-decolorization of more toxic dyes (e.g. RAY). A threshold dose of RAY (approximate to 50 mg l(-1)) is that immediately above which death responses begin to manifest themselves. Decolorization is not growth-associated, but is still metabolism related. Maintenance of cellular viability is thus of importance, since a basic metabolism to express azoreductase activity must be sustained for decolorization. A failure to preserve cellular viability due to chronic toxicity to cells leads to the occurrence of biosorption rather than decolorization. (C) 2002 Elsevier Science Ltd. All rights reserved.