In recent years, accumulated evidence indicates that free radical species and nitric oxide (NO) or its derivatives are the key denominators in carcinogenesis. Our present topics discussed in this article will focus on the biological significance of free radical generation induced by viral and bacterial infections. In influenza virus infection in mice, the level of xanthine oxidase (XO) at the infected sites was elevated to a great extent. The timing of paralleled induction of XO with that of inducible NO synthase (iNOS) indicates efficient simultaneous reaction: NO + O2*- --> ONOO- (peroxynitrite). Peroxynitrite formation was identified by immunostaining of nitrotyrosine at the local site of infected organs. Peroxynitrite exhibits unique chemical reactivities such as protein nitration, DNA-strand breakage, guanine nitration, etc., which may then bring about not only cytotoxic effect but also mutagenesis. Numbers of evidence in vitro and in vivo show that treatment with chemical carcinogens such as carbon tetrachloride and heterocyclic amines also generated superoxide. The chronic inflammatory reactions, e.g., zymosan- and silica-induced granuloma, revealed very similar free radical generation in vivo. In addition, most experimental solid tumors have elevated levels of iNOS in the tumor tissue, and NO thus generated facilitates vascular permeability, which accelerates nutritional supply to the tumor tissue and hence sustains the rapid tumor growth. These circumstantial evidences suggest that inflammatory responses induced by various pathogens would accelerate mutagenesis as well as tissue damage, whereas NO also sustains more effectively solid tumor growth when normal cells are transformed to tumor or carcinoma cells by the host-derived free radical species.