A new member of the anthramycin group of antibiotics1"'115, sibanomicin has been found in the culture filtrate of Micromonospora sp. SF2364. The antibiotic exhibited a marked antitumor activity in mice bearing leukemia P388 cells and a weak activity against Gram-positive bacteria. In this communication, the production, isolation, characterization, structural elucidation and biological properties of the antibiotic are reported.