A purified isolate of an actinomycete identified as ATCC 39366 is capable of producing the antimicrobial compound CL-1957E which also exhibits antitumor properties.The antimicrobial compound CL-1957E is produced by cultivating isolate ATCC 39366 under aerobic conditions in a culture medium containing assimilable sources of carbon and nitrogen until a substantially quantity of the CL-1957E compound is produced, and subsequently isolating the CL-1957E compound.The antibiotic compound CL-1957E and 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.