In this study, Taguchi L-8 experimental design was applied to determine cytotoxic effects of Reactive Blue 33, which is the most toxic azo reactive dye species, on Allium cepa. With this aim, A. cepa test system was performed to achieve targeted experimental design with three factors (concentration of dye, pH and volume) in two different levels. Toxic conditions were determined considering calculated signal-to-noise ratios. "Smaller is better" approach was followed to calculate signal-to-noise ratios as it was aimed to obtain lower root lengths. In the work, toxic effects of azo dye were also predicted by using the Taguchi method. Taguchi model showed that experimental and predicted values were closer to each other demonstrating the success of Taguchi approach. (C) 2013 Elsevier Inc. All rights reserved.