m.w. 518 m/e-; m.p. 305-310 "C (dec.); uvmax 225.307 nm) possesses a strong antimicrobial activity against grampositive bacteria, including strains which produce inactivating enzymes for some commercial antibiotics. Maduramycin forms a complex with serum albumin, but no complex formation with DNA was observed using absorption spectroscopic and polarographic methods. Maduramycin additionally inhibits the action of some enzymes. The LD,, of maduramycin in mice was > 250 mg/ kg on intraperitoneal administration. Fermentation, isolation, and some of the chemical and biological properties of this new antibiotic are described.