The submitted review summarizes contemporary knowledge of the enzyme P 450, the importance of which in human metabolic processes has been beyond doubt for a long time. The existence of various forms of the enzyme (there is a number of enzymatic forms of this haemoprotein which does not differ only as regards the structure of the alipoprotein but also as regards some physical and chemical properties) was proved in many prokaryotic and eukaryotic organisms. It played a special role in herbivores. New forms were detected which can be induced e. g. by polycyclic hydrocarbons, barbiturates, steroids, ethanol (1), hypolipidaemic substances and some antibiotics. Also other regulating factors are known. The clinical use of P 450 is complicated so far by the great genetically conditioned multiplicity of its forms in man. However, a number of very promising and important applications are foreseen.