A purified isolate of an actinomycete identified as ATCC 39366 is capable of producing the anti-microbial compound CL-1957B which also exhibits antitumor properties.The antimicrobial compound CL-1957B is produced by cultivating isolate ATCC 39366 under aerobic conditions in a culture medium containing assimilable sources of carbon and nitrogen until a substantial quantity of the CL-1957B compound is produced, and subsequently isolating the compound.The antibiotic compound CL-1957B, its pharmaceutically acceptable salts, the pharmaceutical compositions comprising this substance together with a pharmaceutically acceptable carrier is also disclosed, as are methods of treating microbial infections and tumors in mammals, employing these pharmaceutical compositions.