For many years we have known that dental caries is one of the most common diseases of mankind. Only few people have sound teeth till the end of life. Formerly we thought that the only possibility to keep our teeth in good state was to go to the dentist regularly. Since we know how complicated our nutrition is we know too that we have to regard the composition of our food as a principal factor influencing the state of our teeth. The trace-elements e. g. play an important role. Studies with tracers during the last decades have shown that the minerals are metabolized very actively by living organisms. Thus research in our laboratory, together with Prof. SIZOO and Prof. OOLS, has demon strated that intravenously injected radio-active phosphorus has disap peared from the blood already half an hour after injection. Even the hard dental substance participates in this active metabolism. von Jansen, B. C. P.