Cytochrome P450 is a generic name applied to a group of hemoproteins. These enzymes belong to the most significant involved in the metabolism of many xenobiotics (pharmaceuticals or their additives, environmental pollutants), several natural products and endogenous compounds. They catalyze oxidation, peroxidation and reduction reactions, which are involved in the first (so-called derivative) phase of biotransformation of the above-mentioned compounds. Humans are known to widely differ in levels of individual cytochromes P450 and in rates of metabolism of xenobiotics and other substrates of these enzymes. Differences can clearly be related to those in biological response to chemicals and their toxicity. This review is focused mainly on significance of individual forms of cytochrome P450 in reactions, that lead to activation of protoxicants and procarcinogens to their ultimate derivatives. Recent reports on the involvement of cytochromes P450 in processes of chemical carcinogenesis are discussed.