The production and isolation of a new toxic substance, Teleocidin, and its biological properties were previously reported1,2). Thereafter it has been found that an other strain of Streptomyces produced such specific toxic substance as Teleocidin in its cultured mycellium. Comparative tests of these two purified crystalline powders showed the new toxic substance resembles Teleocidin closely though differs in certain chemical properties. Therefore, the original Teleocidin is designated Teleocidin A, whereas that produced by a new strain of Streptomyces is named Teleocidin B, which had been tentatively called as the SK-toxic substance. From the results of the chemical studies of Teleocidin B and its hydrogenated derivative, which was easily obtained as a crystalline form by the catalytic hydrogenation of Teleocidin B with Adam’s catalyst, molecular formula, C28H39~41N3O2 was postulated for Teleocidin B. It was also recognized that an alcoholic hydroxyl, a lactam ring and a heterocyclic ring like indole or pyrrole structure existed as the functional groups of Teleocidin B. A new toxic substance, Teleocidin B, was isolated from the mycellium of a Streptomyces. The chemical structure of this compound was studied using its crystalline hydrogenated derivative (Hydroteleocidin B). As the results of these studies, a new acid anhydride named teleocidic anhydride was isolated by the oxidative degradation and it was postulated as 3,6-dimethyl-6-amyl-l-cyclohexene-l,2-dicarboxylic anhydride. The partial structure of Hy-droteleocidin B was also discussed in this paper.