A novel antitumor antibiotic complex has been obtained from the culture broth of Actinomycete strain DO-11 (KY11091) isolated from a soil sample collected in Sendai, Miyagi, Japan. On the taxonomic studies the producing organism is described as Micromonospora chalcea KY11091. For the production of the antibiotics, soluble starch served as a good carbon source and yeast extract was the best nitrogen source tested. The antibiotic complex designated as tetrocarcins is active against Gram-positive bacteria, but is not active against Gram-negative bacteria. Tetrocarcin A showed bacteriocidal activity against Bacillus subtilis.