The radioprotective effect of hesperidin against genotoxicity induced by gamma-irradiation has been investigated in vivo/in vitro in cultured blood lymphocytes from human volunteers. Peripheral blood samples were collected at 0 (10 min before) and at 1, 2 and 3 h after a single oral ingestion of 250 mg hesperidin. At each time point, the whole blood was exposed in vitro to 150 cGy of (60)Co gamma-irradiation and then the lymphocytes were cultured with mitogenic stimulation to determine the micronuclei in cytokinesis-blocked binucleated cells. For each volunteer, the results showed a significant increase in the incidence of micronuclei after exposure of cells to gamma-irradiation as compared to control samples. The lymphocytes in the blood samples collected at 1 h after hesperidin ingestion and exposed in vitro to gamma-rays exhibited a significant decrease in the incidence of micronuclei, compared with similarly irradiated lymphocytes from blood samples collected at 0 h. The maximum protection and decrease in frequency of micronuclei (33%) was observed at 1 h after ingestion of hesperidin. These data have important application for the protection of human lymphocytes from the genetic damage and side effects induced by gamma-irradiation in patients undergoing radiotherapy.