Tunicamycin is a new antiviral antibiotic produced by Streptomyces lyso superificus, nov. sp. The active substance in culture broth is purified by solvent extraction followed by chromatography on silicic acid. It has a mole cular weight of about 870. The antibiotic is active against animal and plant viruses, Gram-positive bacteria, yeast and fungi. Tunicamycin is similar in some properties to a few known antibiotics but differs from them in certain special characteristics.