The effects of tiazofurin (TR) on proliferation and cytokine-induced nitric oxide (NO) production in the L929 fibrosarcoma cell line and murine embryonic fibroblasts were investigated. Treatment with TR inhibited the growth of nonconfluent L929 cells in a dose-dependent manner. TR, at concentrations not affecting cell viability or proliferation, markedly decreased IFN-gamma + LPS-induced expression of inducible NO synthase (iNOS) mRNA and, subsequently, NO production in confluent L929 cultures. However, TR did not interfere with the IFN-gamma-triggered expression of mRNA for IRF-1, an important iNOS transcription factor, implying that TR interferes with some other intracellular pathway involved in iNOS induction triggered by IFN-gamma + LPS. In contrast to the results obtained in L929 cells, iNOS mRNA expression induced by IFN-gamma + LPS in murine embryonic fibroblasts was resistant to TR, indicating a tumor-selective action of this agent.