Abstract: Zhengguangmycin, an antitumor antibiotic complex consisting of more than ten components, is produced by str. Verticillus vat pingyangensis n. var. As reported previously, two major components of this complex are A₂ and B₂. However, during the course of its manufacture a change has taken place in the constitution of the zhengguangmycin complex, the predominant component obtained from CM-sephadex-C-25 column chromatography was found to correspond to zhengguangmycin A₅ according to its elution sequence. This component was isolated and purified, and was named pingyangmycin. Based on its physicochemical properties, the results of comparison of its terminal amine moiety with spermidine and ¹³C NMR spectrum analysis, the identity of this component. with bleomycin A₅ was confirmed. In comparison with zhengguangmycin A₂ (in mice), pingyangmycin appeared to be less toxic in acute toxicity experiments and less injurious to the lung, a much higher concentration was attained in the esophageal carcinoma tissue and a much stronger inhibitory effect was obtained against transplantable esophageal carcinoma SGA-73. Clinical studies in 349 cases of various kinds of cancer showed that pengyangmycin was markedly effective against squamous cell carcinoma of the head, neck and the cervix and fairly effective against mammary, nasopharyngeal and esophageal carcinoma. In View of the fact that bleomycin obtained from commercial sources contains A₂ as its main component with only trace amount of bleomycin A₅, pingyangmycin would be more interesting as an antitumor agent. This paper deals with the results of isolation, physicochemical properties and identification of pingyangmycin.