This study is a continuation of our previous work [Murugan, S.S., Balakrishnamurthy, P., Mathew, Y.J., 2007. Antimutagenic effect of broccoli flower head by the Ames Salmonella reverse mutation assay. Phytother. Res. 21, 545-547], in search of possible antimutagenic properties in broccoli flower head extracts. In the present investigation, the effect of addition of ethanol extract of broccoli flower head on mitomycin-C (MMC) induced sister chromatid exchange (SCE) in cultured human peripheral blood lymphocytes was investigated. Broccoli flower head was extracted in ethanol using either acetone or ethanol as solvents. The extract was tested at final concentrations of 200 and 400 microg/ml culture and set for SCE assay. MMC at a concentration of 1 microg/ml and the test concentrations of broccoli flower head were added to the culture following 48 h from the initiation of culture. Enumeration of SCE in second division mitotic cells indicated that broccoli flower head extract significantly reduced MMC induced SCEs at both the concentrations tested. This observation is in line with our earlier finding and confirms to the presence of antimutagenic principles in broccoli flower head extract.