In the research for new antitumor substances produced by Streptomyces especially active against Ehrlich ascites carcinoma, the authors isolated a new strain No. 48-B-3 from the soil of Akihabara, Tokyo. This strain produces a substance which shows a specific activity against Ehrlich ascites carcinoma and Yoshida sarcoma. The authors obtained this substance in crude powder and designated it as Carzinocidin. In this paper, extraction and chemical and biological properties of this substance are reported.