We recently reported development of an experimental model for the study of nitric oxide (NO^·) toxicology in vivo. SJL mice were injected with superantigen-bearing RcsX (pre-B-cell lymphoma) cells, which migrated to the spleen and lymph nodes, where their rapid growth induced activation of macrophages to produce large amounts of NO^· over a period of several weeks. In the experiments described here, we used this model to investigate mutagenesis in splenocytes exposed to NO^· during RcsX cell growth. Transgenic mice were produced by crossbreeding animals of the pUR288 transgenic C57BL/6 and SJL strains. RcsX cells were injected into F₁ mice and NO^· production was confirmed by quantification of urinary nitrate, the ultimate metabolite of NO^·. Mutant frequency in the lacZ gene of the pUR288 plasmid was determined in DNA isolated from spleen (target) and kidney (nontarget) tissues. A significant elevation in mutant frequency was found in the spleen, but not in the kidney, of tumor-bearing mice. Furthermore, increases in mutant frequency in the spleen as well as NO^· production were abrogated by administration of N-methylarginine, a NO^· inhibitor, to mice following injection of RcsX cells. These results indicate that NO^· had mutagenic activity in RcsX tumor-bearing mice and thus support a possible role for its involvement in the carcinogenic process.