We isolated novel antitumor antibiotics designated as tetrocarcins A, B and C from a broth culture of Micromonospora chalcea KY11091 isolated from a soil sample collected in Sendai-shi, Miyagi, Japan. The seed flasks were inoculated with stock cultures maintained in a deep freezer (-70°C) and grown for 48-72 hours at 28°C, followed by fermentation in a 30-liter jar fermentor at 30°C for 72-96 hours. The culture filtrate was absorbed on nonporous resin HP-20, eluted with acetone, passed through a charcoal column, and subjected to silica gel chromatography to obtain tetrocarcins A, B and C. Tetrocarcin A was confirmed to be identical with antlermicin A by spectral data (UV, IR, PMR, CMR) and various thin-layer chromatographies. Hydrolysis of tetrocarcin A by mild acid gave sugars A (identified as L-amicetose) and B (identified as L-digitoxose) and a component f-1. Tetrocarcins A, B and C showed antibacterial activity (Table 3) and antitumor activity against mouse sarcoma 180 and mouse leukemia P388 (Table 4). The LD50 values of tetrocarcins in mice were 60~80 mg/kg of body weight by intraperitoneal injection, and they caused little reduction in the number of leucocytes in mice during treatments.