Cyclophosphamide (CP) is a widely used antineoplastic drug that causes toxicity in the normal cell due to its metabolites. The major drawback of this drug is an undesirable myelosuppression. Selenium (Se) is a potent nutritional antioxidant that carries out biological effects by its incorporation into selenoproteins, such as glutathione peroxidase (GPx). The possible protective effects of seleno-L-methionine (SLM) against CP-related toxicity of blood cells and bone marrow of rats were investigated in this study. Intraperitoneal (i.p) administration of 50, 100, or 150 mg/kg of CP caused, in a dose-dependent manner, reductions in the number of leukocytes (78, 89, and 92%, respectively), thrombocytes (22, 33, and 52%, respectively), and bone marrow-nucleated cells (72, 90, and 94%, respectively). The groups that had CP treatment alone were killed 3 days after the CP injection. For the groups having CP+SLM, SLM (0.4 or 0.8 mg/kg i.p) administration was started 3 days earlier than the CP administration and continued to the end of the experiment (6 days). On day 4, the animals were weighed again, relative doses of CP were estimated, and CP+SLM was administered together. On day 7, blood samples were collected and bone marrow of animals were resected under anesthesia. The results indicated that treatment of rats within a select dose range of SLM could reduce CP-induced toxicity on blood cells and bone marrow.