In an earlier report 1 l, the isolation and properties of tylosin were described. In the fermentation which produces tylosin, other antibiotics are synthesized and can be detected on bioautograph plates after paper chromatography of the broth filtrate. This paper decribes the fermentation, isolation, physicochemical properties, and biological activities of macrocin, a second antibiotic found in the culture broth which produces tylosin. Lactenocin, a microbiologically active substance, can be prepared by the degradation of macrocin.