Here we report an in vitro investigation concerning some protective properties of a whey protein isolate (WPI), a commercially available product obtained frommilk by ion exchange and ultrafiltration. The study aim was the evaluation of WPI cellular toxicity, product influence on doxorubicin (Dox) induced toxicity in cells pretreated with WPI and on the DNA damage produced by ã-rays in human lymphocytes. WPI practically showed no cytotoxicity within a large interval of concentrations (900-9 ìg cys/ml). In associated treatments with Dox,WPI at doses of 900, 180 and 90 ìg cys/ml attenuated the drug cytotoxic sideeffects in normal cells (Hfl-1) in a dose linear dependent manner. ã-rays induced genotoxicity was significantly reduced by WPI, particularly by accelerating the DNA repair of the radioinduced damage in human lymphocytes, in a dose-effect relationship. Significant differences were found in the number of repaired lesions (p < 0.01), at 2 hrs (2.93, 2.06 and 1.98 fold lower at doses of 450, 180 and 90 ìg cys/ml, respectively).