The authors present a test procedure which permits to obtain, by means of a single carbon-14-labelled compound as structural element in a series of essential substances, a survey of important fields of liver function. Special attention was paid to cholesterol. Methodical studies resulted in an optimization of the application under in vivo and in vitro conditions, and of the isolation of carbon-14-labelled cholesterol, ester cholesterol and glucose as well as of carbon-14-labelled phospholipids, triglycerides, bile acids, proteins and nucleic acids as groups. A comparison of the experimental techniques demonstrated the advantages of in vivo application. The method and its validity are illustrated by an example (DDT). Its use in studies on various active principles will be dealt with in future communications.