Flax pulp was bleached by using an enzyme treatment with laccase (L stage) in the presence of HBT as mediator in order to replace chlorine-based pulp bleaching processes which produce toxic organochlorine compounds with biotechnological products, achieving an environmentally friendly process technology. Enzyme treatments were sequentially conducted according to a factorial design involving four variables: laccase dose, HBT dose, treatment time and oxygen pressure in the reactor. Their influence on the effluent properties (COD, color, toxicity, spectra and residual enzymatic activity) was examined to evaluate the impact of L stage on environment since no experimental data have been published concerning this matter. Mathematical models accurately predicting effluent properties in terms of the previous four variables were developed. High COD levels were obtained as a result of using commercial laccase. Also, red color produced, especially at long treatment time, relates to formation of oxidation products from HBT. The residual enzymatic activity depends basically on mediator dose, and mainly activity loss is produced during the first 30 min of treatment. The toxicity of the effluents was below the limits set by the sewage regulations for Catalonia and can be ascribed to the combined effect of the laccase-mediator system. (C) 2009 Elsevier B.V. All rights reserved.