In the course of our search for novel antitumor antibiotics from microorganisms, Streptomyces sp. HP530 was found to produce two potent antitumor antibiotics, saptomycins D and E. Chemical structures of these compounds are closely related to those of the pluramycin-group antibiotics, which have remarkable antitumor activities. In this communication, we describe the production, purification, physico-chemical characterization, structure determination and biological properties of saptomycins D and E.