Inhibitory effects of balsam pear on the mutagenic activity of cyclophosphamide were studied in the cells of bone marrow of mouse in vivo. It was found that balsam pear juice itself showed no effect on the incidence of sister chromatid exchanges (SCE) and of micronuclei (MN) on the cells of bone marrow. But balsam pear juice reduced the incidence of SCE (from 29.1 times/cell to 13.27-28.38 times/cell) and MN (from 60.0% to 27.0%-50.0%) induced by cyclophosphamide respectively. The inhibitory effects showed a dose-dependent relation, which was statistically significant.