Neoxanthin, one of the major carotenoids in fresh spinach, was isolated and purified to its homogeneous form. The cytotoxicity of neoxanthin in C3H10T1/2 cells was investigated by measuring its relative plating efficiency (RPE). At lower concentrations (10^(-1)~10^(-3)μ)g/ml, neoxanthin slightly enhanced RPE (up to 126-130%), When the concentration was increased to 1μg/ml, neoxanthin exerted a strong inhibition on cell growth, with RPE at 15%. In a constructed xanthine oxidase and xanthine system, neoxanthin showed remarkable inhibition on malondialdehyde formation from human RBC membrane. Neoxanthin is 100-fold more ac live than β-carotene in its anti-oxidation activity. Using tritium labeled thymidine, uridine and leucine as markers, neoxanthin could inhibit the incorporation of 3H-lhymidine into DNA, but it could not affect incorporation of 3H-uridine and 3H-leucine into RNA and protein, respectively. Based on these findings, we suggest that neoxanthin might be a promising agent in cancer chemoprevention.