Androgens influence some immunological processes, including alternation of the number and function of the circulating lymphocytes and monocytes. In the present study, the effects of three different doses of testosterone on the numbers and percentages of the peripheral blood cells were investigated; the lymphocyte subsets were determined and the proliferation of lymphocyte was detected. Groups of Sprague-Dawley rats were treated with 0.5, 2.5, 12.5 mg/kg or only vehicle, respectively. Compared with controls, the results of complete blood counts showed that the absolute and relative numbers of monocytes decreased. The lymphocyte subpopulations determined by flow cytometry indicated an increase in CD8+ T cells, whereas the CD3+, CD4+ and CD4+CD8+ T cells remained unchanged. Thus, the immunoregulatory index (CD4+/CD8+ ratio) decreased. The proliferative activities determined by MTT assay were down-regulated. In conclusion, the immunosuppressive effects of testosterone may be attributed to a decline in number of monocytes, CD4+/CD8+ ratio and proliferative activities together with an increase of CD8+ T cells in Sprague-Dawley rats.