The effect of thymol (TOH), a dietary compound was investigated for its ability to protect against radiation-induced cytotoxicity in Chinese hamster lung fibroblast (V79) cells growing in vitro. Treatment of V79 cells with 25 microg/ml of TOH prior to 10 Gy gamma radiation resulted increase in the cell viability than that of radiation alone as evaluated by MTT assay. Similarly, there was a significant increase in the surviving fraction observed with 25 microg/ml of TOH administered 1h prior to graded doses of gamma radiation. Further, 25 microg/ml TOH treatment before irradiation significantly decreased the percentage of radiation-induced apoptotic cells (sub-G(1) population) analyzed by flow cytometry as well as DNA ladder assay. TOH was found to inhibit various free radicals generated in vitro, viz., DPPH, O(2), ABTS(+) and OH in a concentration dependent manner. TOH also inhibited the radiation-induced decrease in intracellular glutathione, superoxide dismutase and catalase enzyme levels in V79 cells accompanied by the reduction in lipid peroxides. Our study demonstrated antagonistic potential of TOH against radiation-induced oxidative stress, lipid peroxidation resulting in increased cell viability.