The cytochrome P450(P450) are a large group of constitutive and inducible heme-containing enzymes, which have a central role in the oxidative metabolism of a diverse range of xenobiotics. The majority of chemical carcinogens require metabolic activation before they interact with cellular macromolecules and can cause cancer initiation. The xenobiotic-metabolizing machinery contains two main types of enzymes : the phase I P450 mediating oxidative metabolism, and phase II containing enzymes. Activity of some enzymes implicated in the metabolism of carcinogens presents a great variability between individuals due to the existence of a polymorphism in gene coding for P450. Individual P450s, especially CYP1B1, are overexpressed in different types of tumors. The increased expressons of P450s in tumors is highly significant and is important for understanding rumor development and progression. The tumor-specific expression of P450s provides the basis for the development of movel diagnostic and therapeutic strategies.