A Streptomyces species, originally designated as No. 751, isolated from soil, was carefully studied from the standpoint of its morphological, physiological, and cultural characteristics. Because it produces soluble, deep purplish-red pigment in mixed culture with various microorganisms, but does not produce such pigment when cultivated independently, it is proposed as a new species and is designated Streptomyces propurpuratus nov. sp. SHINOBU et KANDA. The type strain is designated as OEU No. 751. It is not difficult to imagine that, in nature, a product of metabolism of a microorganism might be changed by the effect of a product of metabolism of another microorganism, whether the reactions are chemical or enzymatic. The authors believe that such phenomenaoccur not only with regard to pigment formation, but also with regard to production of antibiotics. An example of such a phenomenon is reported herein.