The MeOH extract of the fruits of Melia toosendan was selected for futher study by its cytotoxicity and effect on the human breast cancer cell line, MCF-7. The active principle obtained by activity guided fractionation followed by purification gave rise to a needle crystal. The structure was deduced by employing NMR and was determined to be identical with 28-deacetyl sendanin by comparison with published data. This compound induced morphological change of MCF-7 to be rounded with tubule at concentrations between $50;{mu}g/ml$ and $0.025;{mu}g/ml$. This compound, however, showed strong cytotoxic effect on Hepalclc7 and HepG2, and their $GI_{50}$ on the hepatoma cell lines were $0.238;{mu}g/ml$ and $0.805;{mu}g/ml$, respectively. Its effect on lymphocyte of mouse was stronger than hepatoma cell lines, and their $ED_{50}$ of polyclonal antibody response was $0.011;{mu}g/ml$, and $ED_{50}$ of cell viability was $0.039;{mu}g/ml$.