A new antibiotic, tuberactinomycin ( TUM ) has been isolated from the broth filtrate of Streptomyces griseoverticillatus var. tuberacticus obtained from a soil sample at Ohito-cho, Shizuoka prefecture (1). TUM is a peptide antibiotic effective against tubercular bacilli (2). The structural study of this antibiotic started with isolation and characterization of amino acid components. Acid hydrolyzate of TUM was chromatographed on Dowex-5OWxZ (200-400 mesh, Ht form) column, and eluted with a buffer solution of 0.2M pyridine and formic acid (pH 3.1). From the column, L-serine, L-a,s-diaminopropionic acid, guanidino amino acid and T-hydroxy-l-slysine (3) were eluted successively. Although an original eluate containing the guanidino amino acid gave positive reactions both for ninhydrin and Sakaguchi reagents, the crystals of fine prisms obtained through concentration in vacua were negative for both reactions, m.p. 245O(decomp.). Molecular formula of --C~~H,~N,OS(I) was deduced to this compound from the results of elementary analysis of I and molecular ?eak(480) in the mass spectrum of its diacetyl derivative (4). When l.lg of the compound I was treated with 4.5ml of 15% hydrobromic acid at 50", it was degradated to two compounds (II and III) which were isolated separately by fractional crystallization with aqueous ethanol in a yield of 400mg and 320mg respectively.