BACKGROUND Effectiveness of the immune defense formed by the genotype often determines the predisposition to cancer. Nitric oxide (NO) produced by macrophages is an important element in this defense. MATERIAL AND METHODS We hypothesized that genetic characteristics of NO generation systems can predetermine the vulnerability to tumor development. The study was conducted on mice of 2 genetic substrains - C57BL/6J and C57BL/6N - with Ehrlich ascites carcinoma (EAC). NO production in the tumor was changed using ITU, an iNOS inhibitor; c-PTIO, a NO scavenger; and SNP, a NO donor. Macrophage NO production was estimated by nitrite concentration in the culture medium. iNOS content was measured by Western blot analysis. Macrophage phenotype was determined by changes in NO production, iNOS level, and CD markers of the phenotype. RESULTS The lifespan of C57BL/6N mice (n=10) with EAC was 25% longer (p<0.01) than in C57BL/6J mice (n=10). Decreased NO production 23% reduced the survival duration of C57BL/6N mice (p<0.05), which were more resistant to tumors. Elevated NO production 26% increased the survival duration of C57BL/6J mice (p<0.05), which were more susceptible to EAC. Both the NO production and the iNOS level were 1.5 times higher in C57BL/6N than in C57BL/6J mice (p<0.01). CD markers confirmed that C57BL/6N macrophages had the M1 and C57BL/6J macrophages had the M2 phenotype. CONCLUSIONS The vulnerability to the tumor development can be predetermined by genetic characteristics of the NO generation system in macrophages. The important role of NO in anti-EAC immunity should be taken into account in elaboration of new antitumor therapies.