We have previously observed that ionizing radiation induces tyrosine phosphorylation in human B-lymphocyte precursors by stimulation of unidentified tyrosine kinases and this phosphorylation is substantially augmented by vanadate. Ionizing radiation generates reactive oxygen intermediates (ROI). Because H2O2 is a potent ROI generator that readily crosses the plasma membrane, we used H2O2 to examine the effects of ROI on signal transduction. We now provide evidence that the tyrosine kinase inhibitor herbimycin A and the free radical scavenger N- acetyl-cysteine inhibit both radiation-induced and H2O2-induced activation of NF-kappa B, indicating that activation triggered by ROI is dependent on tyrosine kinase activity. H2O2 was found to stimulate Ins-1,4,5-P3 production in a tyrosine kinase-dependent manner and to induce calcium signals that were greatly augmented by vanadate. The synergistic induction of tyrosine phosphorylation by H2O2 plus vanadate included physiologically relevant proteins such as PLC gamma 1. Although treatment of cells with H2O2 alone did not affect the activity of src family kinases, treatment with H2O2 plus vanadate led to activation of the p56lck and p59fyn tyrosine kinases. The combined inhibition of phosphatases and activation of kinases provides a potent mechanism for the synergistic effects of H2O2 plus vanadate. Induction of tyrosine phosphorylation by ROI may thus lead to many of the pleiotropic effects of ROI in lymphoid cells, including downstream activation of PLC gamma 1 and NF-kappa B.