The effects of Cordyceps cicadae extracted fractions on human mononuclear cells (HMNC) proliferation were determined by tritiated thymidine uptake. The results indicated that aqueous methanol (50%) extracts of C. cicadae ascocarps portion (CC-1-2) enhanced HMNC proliferation activated with phytohemagglutinin (PHA) with an EC(50) of 13.8+/-4.6 micro g/ml. By contrast, the methanol (100%) extracts of C. cicadae insect-body portion (CC-2-1) suppressed HMNC proliferation stimulated by PHA with an IC(50) of 32.5+/-5.2 micro g/ml. Cell viability test indicated that inhibitory effects of CC-2-1 on HMNC proliferation were not through direct cytotoxicity. The action mechanisms of CC-1-2 and CC-2-1 may involve the regulation of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) production in HMNC. Since CC-2-1 suppressed IL-2 and IFN-gamma production in HMNC induced with PHA. The CC-1-2 enhanced IL-2 and IFN-gamma production of HMNC stimulated with PHA in a concentration dependent manner. Therefore, the results demonstrated that C. cicadae contained growth modulators for HMNC.