Three new quassinoids, bruceanols D [1], E [2], and F [3], were isolated from Brucea antidysenterica, and their structures were elucidated by spectral evidence and chemical transformation. All of these compounds exhibited cytotoxicity against five human tumor cell lines, malignant melanoma (RPMI-7951), lung carcinoma (A-549), ileocecal adenocarcinoma (HCT-8), epidermoid carcinoma of the nasopharynx (KB), and medulloblastoma (TE-671), and against murine lymphocytic leukemia (P-388).