Flax Pulp was bleached by using an enzyme treatment with laccase (L stage) in the presence of HBT as mediator in order to replace toxic chemicals (e.g. chlorine or chlorine dioxide) with environmentally friendly catalytic biotechnological products in the bleaching process, achieving a clean process technology in the pulp and paper manufacture. The operating conditions for the laccase-HBT system were optimized by using a sequential statistical plan involving four variables (laccase dose. HBT dose, treatment time and oxygen pressure), their influence on the properties or the pulp after the L stage was examined. The main objective was to minimize the reagent doses and the reaction time to make more suitable the industrial application. A mathematical model for predicting the kappa number in terms of the process variables was developed. The kappa number decreased with increase in the value of each variable down to a minimum level of 6.9, which was obtained with a laccase dose of 13 U g(-1), an HBT dose of 2%odp and a treatment time of 6 5 h.. The model predicted a limiting laccase dose of 13 U g(-1) above which no additional reduction in kappa number would be obtained. By contrast, oxygen pressures of 0.2-0.6 MPa in the reactor had no effect on brightness or kappa number. Brightness was not correlated with the kappa number due to the formation of chromophores in the pulp at the beginning of the enzymatic treatment. The LP biochemical sequence allows obtaining a final brightness of 75.2%ISO. (C) 2009 Elsevier B.V. All rights reserved.