The present study is a pilot study that explores the antioxidative properties of linseed (Linum usitatissimum) oil in its prophylactic action against oxidative stress induced by a radiomimetic drug, cyclophosphamide. Oral administration of linseed oil (0.1 mL/kg of body weight/day) for 20 days prior to an acute dose of cyclophosphamide (75 mg/kg) significantly inhibited the augmented level of malondialdehyde, conjugated dienes, and hydroperoxides in the mouse brain. The cyclophosphamide- induced decline in the levels of reduced glutathione, glutathione peroxidase, and alkaline phosphatase was also significantly prevented by linseed oil in mouse blood. Similarly, the increased activity of acid phosphatase and oxidized glutathione was significantly inhibited by linseed oil. Results clearly indicate the prophylactic action of linseed oil against cyclophosphamide- induced oxidative stress.