The microbiological characteristics of Streptomyces strain No. 7906 include formation of white aerial mycelium, light brown growth and thick aerial hyphae forming tufts. Strain No. 7906 was compared with known species and identified as a new species and given the name Streptomyces tateyamensis. Streptomyces tateyamensis No. 7906 produces new sulfur-containing peptide antibiotics which are named thiopeptins. The major component, thiopeptin B, is obtained as a crystalline powder decomposing at 219-222°C, and exhibiting optical rotation [a]23D -80°(c 1, chloroform). Its elementary analysis suggested the empirical formula C72H90O22N18S6. The ultraviolet absorption maxima (shoulder) were found at 230-250, 295 and 305 mμ. Thiopeptin B showed strong antibacterial activity against Gram-positive bacteria and had no cross resistance with penicillins, aminoglycoside antibiotics, tetracyclines and macrolides. The administration of 500 mg/kg of this antibiotic into mice by intraperitoneal route did not result in any toxic symptom.