Cell culture and treatmentsChinese hamster ovary cells (CHO-9 cell line) were kindly supplied by Prof. A.T. Natarajan (University of Leiden, The Netherlands). Cells were maintained as monolayers growing at 37°C in 25-cm 2 flasks (Corning) containing HAM-F10 (Sigma) plus DMEM (Dulbecco's modi- ABSTRACT Naturally occurring antioxidants have been extensively studied for their capacity to protect organisms and cells from oxidative damage. Many plant constituents including turmeric and curcumin appear to be potent antimutagens and antioxidants. The effects of turmeric and curcumin on chromosomal aberration frequencies induced by the radiomimetic agent bleomycin (BLM) were investigated in Chinese hamster ovary (CHO) cells. Three concentrations of each drug, turmeric (100, 250 and 500 µg/ ml) and curcumin (2.5, 5 and 10 µg/ml), were combined with BLM (10 µg/ml) in CHO cells treated during the G 1 /S, S or G 2 /S phases of the cell cycle. Neither turmeric nor curcumin prevented BLM-induced chromosomal damage in any phases of the cell cycle. Conversely, a potentiation of the clastogenicity of BLM by curcumin was clearly observed in cells treated during the S and G 2 /S phases. Curcumin was also clastogenic by itself at 10 µg/ml in two protocols used. However, the exact mechanism by which curcumin produced clastogenic and potentiating effects remains unknown.