In the course of a continuing search for tumor inhibitors of plant origin, an alcoholic extract of the stem bark of Bersama abyssinica Fresen. (Melianthaceae), collected in Ethiopia in March 1965, was found to show significant inhibitory activity against cells derived from human carcinoma of the nasopharynx carried in cell culture (KB). We report herein the isolation and characterization of hellebrigenin 3-acetate (I) and hellebrigenin 3,5-diacetate (II), two novel naturally-occurring tumor inhibitors from B. abyssinica. The two hellebrigenin acetates appear to be the first recognized naturally-occurring bufadienolide 3-esters; the 3,5-diacetate appears to be the first reported 5β-acetoxy steroid. Furthermore, although other cardenolides (1-3) and bufadienolides (3) have been found earlier to possess marked cytotoxic activity against KB tissue culture, hellebrigenin 3-acetate appears to be the first cardiotonic steroid recognized to show significant and reproducible inhibitory activity against an in vivo tumor system. Hellebrigenin 3-acetate and hellebrigenin 3,5-diacetate showed significant cytotoxicity (ED50) against KB cell culture at 10^-4 and 10^-3 µg./ml., respectively. Hellebrigenin 3-acetate showed significant inhibitory activity against intramuscular Walker carcinosarcoma 256 in rats at 8 mg/kg.