I APPROACHED the study of the production of actinomycin by Streptomyces antibioticus No. 105/5 from the point of view of the biosynthesis of a model antibiotic with a quinone structure. I thought that the biosynthesis of actinomycin, as a quinone antibiotic, might be catalysed by phenol oxidases. Although the molecule of actinomycin is not a typical quinone with two carbonyl groups, phenol oxidases may take a part in the biosynthesis of this antibiotic, at least during the formation of some intermediary product. For example, dioxi-dimethylacridonquinone and dioxitoluquinone were obtained during the breakdown of actinomycin1.