Halvolio acid (Ia) (1) is a fusidana type antibiotic like fusidic acid (IIa) (2) and cephalosporin PI (III) (3). Recent publications regarding Ia and III have prompted us to publish the accumulated data*' on the stereochemistry of this compound. TLiE POSITION OF lh-CH? Previously re established framework (A) for helvolic acid in which the third tert. CRs was tentatively proposed to be located at Cl4 (lb). Its definite proof has been obtained by the en01 acetylation of methyl tetrahydrohelvolate (IVa) which afforded two enol acetates (V, VI). Vpm.p.207-8: W*'219 (3.85)) EbiR*"3 COC&(7.85,7.90,8.03),Ca-U(ir.65 ca., overlaped with Cv-H signal), C,-H(&.66,s),C1s -H(h.ll,diffused d,J=8 ca.). VI, m.p.175: W 269(h.17), RER 2 COa(7.89,7.9h),Cs-S(&.76 ca., overlaped with CT-H signal),Cv-H(~.70,s),C~s-and Cl,-E(3.06,3.23,d,J-6). BMFi spectrum of VI demonstrates that a double bond generated by an elimination of 16-acetoxyl is A1s and that Cl, possesses no hydrogen, because the signals of Cls-and Crs-H are sharp doublets. Consequently the position of third tert. CHs is proved to be at Cl4 and the fuaidane type framework of Ia thus becomes clear. lhB-CHa Treatment of IO eith zinc dust and acetic acid furnished 'I-desacttoxydihydrohtlvolic acid (VII) (lb,c) which was converted into fi2-17-keto derivative (IX) via VIII, m.p.lRY-92: RER 2 COs(7.81,7.89)Cs-R(5.52,m),C1s-R(h.69,diffueed d,J=ll *I The stereochem. of Ia was partly discussed at 8th symposium on the chemistry of natural products, Nagoya, Japan, l%h( S.Iwasaki, S.Okuda, K.Tsuda, symp. papers ~192). *s All new compounds cited in this paper gave satisfactory analytical data. *a All W spectra,hmax mu (logE), were measured in ethanol solution. *a q.value and abbreviations (s:singltt, d:doublet, used.