Mesenchymal stem cell (MSC) is being used in regenerative medicine and also for the treatment of autoimmune diseases and graft-versus-host disease. Cyclophosphamide (CTX) is a chemotherapeutic agent used for the treatment of cancer as well as several immunological diseases. The present study was designed to evaluate the therapeutic role of bone marrow-derived mesenchymal stem cells (BM-MSCs) in ameliorating the adverse effect of cyclophosphamide-induced immunohistochemical toxicity in adult male albino mice. Forty male albino mice of matched age were used for this study. They were divided into four equal groups. Group I served as the control; Group II was treated with cyclophosphamide (CTX); Group III was administered with cyclophosphamide concomitantly with MSCs; while the fourth group was used to isolate bone marrow. Blood leukocyte, splenocyte numbers, blood cell type levels, and splenocyte were determined after one month of initiation of the experiment. Two markers; the cluster of differentiation 3 (CD3) and proliferating cell nuclear antigen (PCNA) were used for immunohistochemical analysis. The results showed that CTX induced a reduction in overall proliferating cells in splenic parenchyma with clear CD3 positive cells in the periarterial sheath. MSCs induced reduction in T-cell number sparing other cells as indicated from intense cell proliferation in the splenic white and red pulp in this group of animals. The study demonstrated that MSCs induced reduction of T-cell number associated with a proliferation of other immune cells of the spleen.