The effects of a cytotoxic drug, cyclophosphamide, and an antioxidant, vitamin E, both individually and also in combination were studied in fibrosarcoma-induced rats. The rate of formation of lipid peroxides was analyzed in untreated, cyclophosphamide- (20mg/kg body weight) and α-tocopherol- (400 mg/kg body weight) administered animals along with that of their respective controls. The levels of non-enzymic antioxidants, like glutathione and vitamin E, and enzymic antioxidants, viz., catalase, superoxide dismutase, and glutathione peroxidase, were analyzed as well. Significantly increased level of lipid peroxides was observed with a concomitant decrease in enzymic and non-enzymic antioxidants in fibrosarcoma-bearing rats when these levels were compared with those of the respective controls. A further increase in lipid peroxides and a decrease in the level of antioxidants were observed in the cyclophosphamide-treated animals. In vitamin E-treated, fibrosarcoma-bearing rats, there was a decrease in lipid peroxide level, and an increase in antioxidant level, probably due to the free radical-quenching activity of vitamin E. When cyclophosphamide was administered in combination with vitamin E to the fibrosarcoma-bearing rats, the corrected levels of these parameters were observed to approach the control values. From these results, we infer that vitamin E is beneficial combinative factor for controlling the increased risk of lipid peroxidation induced by cyclophosphamide. Thus, vitamin E augments the anticarcinogenic activity of cyclophosphamide by decreasing its cytotoxic effect, which is enhanced by lipid peroxidation.