The present paper deals with, first, a review on the penicilin-resistibility of microorganisms, and, second, a report of the results of the experiments which were designed and carried out by the present writer in order to investigate the properties of penicillin-resistant microorganisms. It has been the hope of the present writer, through further studies on this line, to gain a deeper insight into the mechanism of the development of penicillin-resistance and of the penicillin action on microorganisms. Elucidation of these mechanisms may, then, lead to development of a method to prevent aquisition of resistance by mircoorganisms, whereby a more effective penicillin therapy may eventually be established. It was observed in the writer's experiments that the development of penicillinresistance is checked by quinine and acriflavin, which are the metabolic inhibitors of flavoproteins of the respiratory enzymes in microorganisms. According to the writer's interpretation, this phenomenon indicates the important role of flavoproteins in the process of the development of penicillin-resistance in micro-organisms.
