B-lymphocytes express 5-lipoxygenase (5-LO) protein but cellular leukotriene production is suppressed by selenium-dependent peroxidases. Thus it was of interest to check whether reactive oxygen species (ROS) which are released under inflammatory conditions can stimulate B-lymphocyte 5-LO and counteract peroxidase-mediated suppression of cellular 5-LO activity. It was found that 5-LO in the Epstein-Barr virus-transformed B-lymphocytic cell line BL41-E95-A is activated by addition of hydrogen peroxide or xanthine/xanthine oxidase and after increasing the oxidative state of the cell by azodicarboxylic acid bis(dimethylamide). Generation of endogenous ROS from mitochondria by antimycin A also lead to a threefold upregulation of 5-LO activity in B-cells. There was almost no detectable endogenous superoxide formation in BL41-E95-A cells after stimulation with 4beta-phorbol 12-myristate 13-acetate. Co-incubation experiments with BL41-E95-A cells and granulocytes demonstrated that granulocyte-derived ROS can activate B-lymphocyte 5-LO. Addition of superoxide dismutase and/or catalase to the B-lymphocyte/granulocyte co-incubations and to B-lymphocyte homogenates revealed that the 5-LO activation is due to the superoxide-derived release of hydroperoxides or hydrogen peroxide from granulocytes. The data suggest that ROS formation plays an important role in the regulation of cellular 5-LO activity in B-lymphocytes. As leukotrienes affect B-cell functions like cell proliferation, activation and maturation, this finding provides a new link between the formation of ROS and the regulation of immune responses.