A new antibiotic, cyanocycline A, was isolated from the fermentation broth of Streptomyces flavogriseus strain No. 49, a soil isolate. The molecular formula of cyanocycline A was determined to be C22H26N4O5. The antibiotic has a cyano group and a N-heterocyclic quinone moiety in its structure. Cyanocycline A was found to have broad spectrum antimicrobial and antitumor activity. During the course of screening for antibiotics produced by strains of Actinomycetes, a strain of Streptomyces was isolated from a soil sample collected at Tama City, Tokyo. Cultures of this microorganism were found to produce marked antimicrobial and antitumor activity. The antibiotic was isolated in an orange-red crystalline form from the culture filtrate of the streptomycete grown in submerged aerated liquid culture. The IR, UV and NMR spectra suggested that the antibiotic has a cyano group and a benzoquinone moiety. The structure elucidated by X-ray crystallography revealed that the antibiotic belongs to the group of N-heterocyclic quinones. At first the antibiotic was called antibiotic No. 49; it was renamed cyanocycline A, when it was found that its structure has a cyano group and heterocyclic ring. The present paper deals with the taxonomy of the producing strain, and the isolation, characterization and biological properties of cyanocycline A.