Objective To study the influence of blood extracts from pilose antler on the immune system in healthy mice and tumor-bearing mice. Methods A mouse mammary tumor transplantation model was established and both the healthy and the tumor-bearing mice were gavaged with the blood extracts of pilose antler. The percentages of mouse peripheral blood CD3+ T cells, CD4+ T cells, CD8+ T cells, B220+ B cells and myeloid-derived suppressor cells (MDSCs) in the healthy and tumor-bearing mice were analyzed by flow cytometry. Results The blood extracts of pilose antler quantitatively up-regulated the helper T (CD4+) cell numbers, elevated the total T cell and B cell numbers, reduced the MDSC number and inhibited tumor growth. Conclusion The blood extracts of pilose antler can enhance mouse immune functions and inhibit transplanted tumor growth.