Steffimycin is a potent inhibitor of DNA dependent-RNA synthesis patented by Upjohn Co. as an antibiotic. Steffimycin interferes with amino acid incorporation mediated by synthetic polyribonucleotides in cell-free bacterial systems. This inhibition is a secondary activity of the antibiotic, which acts primarily as a suppressor of RNA synthesis in bacteria and bacterial cell-free systems. Inhibition of peptide biosynthesis is apparent only at a drug concentration higher than that necessary for inhibition of RNA synthesis in cell-free systems. In addition to antibiotic activity vs certain gram-positive organisms, Steffimycin inhibits the growth of KB cells and several fungi and shows antiviral activity against herpes simplex in mice