Streptomyces sp. HP530 was found to produce novel antitumor antibiotics, saptomycins, closely related to the pluramycin-group and was further found to mutate frequently. The natural mutant produced several new saptomycins as determined by HPLC analyses. We isolated saptomycins A, B, C1, C2 and F from the parent strain and saptomycins D, E, G and H from the mutant. The saptomycins showed antimicrobial activities and potent antitumor activities against human or murine tumor cell lines in vitro and against Meth A fibrosarcoma in vivo. In particular, saptomycin D was most effective component in vivo of all saptomycins.