A new antibiotic, designated X-5108, produced by a new species of microorganism named Streptomyces goldiniensis var. goldiniensis, has been isolated in essentially pure but amorphous form. Assay methods, and cultivation and isolation procedures for the antibiotic are described. The antibiotic is active in vitro primarily against a number of gram-positive bacteria, whereas in vivo it is especially active against Streptococcus pyogenes infections of mice and as a poultry growth promotant. It shows a very low toxicity in mice with an LD50 of >lg/kg sbc, and >4g/kg p.o. Its empirical formula is C44H62N2O12. Structural studies reveal one methoxyl, one N-methyl and six C-methyl groups. UV, IR and NMRspectra and optical rotation of the pure antibiotic are given. The antibiotic appears to belong to a new chemical class.