This review summarizes results obtained in the author's and collaborating laboratories within the last decade and is designed to attract the attention of researchers to discrete biochemical mechanisms of protection acquired in vivo by cells of malignant tumors against effectors of innate antitumor immunity. Tumor progression in vivo is associated with the appearance and selection of tumor cells with new specific characteristics: a high level of H(2)O(2)-catabolizing (antioxidant) activity (H(2)O(2)CA) and the ability for immediate release of E2-type prostaglandin (PGES) on contact with natural killers, macrophages, and neutrophils; the expression of the [H(2)O(2)CA + PGES] phenotype provides the tumor cells with two mechanisms of local protection against effectors of innate and acquired antitumor immunity. This results in a 10-100-fold less effective rejection of tumor cells in immune and normal animals and corresponding increase of tumorigenicity. The in vitro transformation of normal fibroblasts, spontaneous or induced by oncogenes LTSV40, E1a,b, Ha-ras, myc, and also by p53(175) and bcl-2 does not result in the [H(2)O(2)CA + PGES] phenotype expression, but during subsequent in vivo growth of the above-mentioned transformants the selection of tumor cells of the [H(2)O(2)CA + PGES] phenotype is correlated with a 30-200-fold increase in their tumorigenicity (accompanied or not accompanied by spontaneous metastatic activity). Unlike the transformation induced by the above-mentioned oncogenes, the transformation of normal cells by the v-src gene results in the [H(2)O(2)CA + PGES] phenotype expression. The data presented confirm the determining role of the v-src gene in the expression of the [H(2)O(2)CA + PGES] phenotype. In various primary viral carcinogenesis (SV40, SA7(C8)) the natural selection of cells expressing the [H(2)O(2)CA + PGES] phenotype begins even within the latent period and can be completed by the appearance of primary tumors.