The development of new antitumor drugs is required for cancer treatment because most human tumors are refractory to current conventional therapy. During our continuing search for new antitumor agents from microbial natural products, we discovered two novel depsipeptides belonging to the quinomycin family designated as SW-163C and E from the culture broth of a streptomycete strain SNA15896. The producing organism, SNA15896, was identified as a member of Streptomyces based on its morphological and cultural characteristics. SW-163C and E exhibited potent antitumor activities against various tumor cell lines in vitro and against murine leukemia P388 in vivo. The compounds also showed antimicrobial activities. This paper deals with the taxonomy of the producing strain, fermentation, isolation and biological activities of the compounds.