In oxygen-containing environments, cellular metabolism results in the production of several potentially toxic oxygen-derived molecular species. Accordingly, all normal cells have various defense systems against such active oxygen species. However, several tumor cells are known to have lost part of these defense systems1}. Thus, it could be expected that substances generating active oxygen might show selective cytotoxicity against such tumor cells. In the course of our screening for antitumor antibiotics generating active oxygen in tumor cells, Streptomyces sp. KB10was found to produce two new antibiotics designated menoxymycins A and B.