A new antibiotic, rosamicin, classified as a macrolide, has been isolated from the fermentation broth of a new species of Micromonospora, M. rosaria. The antibiotic is separated from the broth by a solvent extraction procedure and purified by column chromatography. Chromatographic studies indicate that rosamicin is different from all related macrolides and is a novel antibiotic. It has broad-spectrum activity, although it is more potent against gram-positive organisms. Rosamicin is also active against Mycoplasma.