Phellinus linteus (PL)-hot water extract (PL-W) or- methanol extract (PL-M) was orally administered alone (single dose of 400, 800, 1600 mg/kg; 800 mg/kg/day for 5 days) or with cyclophosphamide (CY, 20 mg/kg, i.p.) to female ICR mice. Within PL alone-treated group, WBC and plaque forming cells (PFC) to SRBC were slightly and significantly enhanced when compared with control group. The relative thymus and spleen weights, WBC and PFC numbers were significantly decreased by the treatment of CY, whereas those values were markedly increased by the concomitant treatment of CY and PL when compared with CY administration alone. To assess the effects of PL and/or CY on the mitogen response of splenocytes io LPS, mouse splenocytes were stimulated with or without LPS in the presence of various concentration of PL and/or CY in vitro and splenocytes proliferation (SP) was measured by MTT assay. PL alone increased both SP and LPS- stimulated SP. Moreover, SP and LPS-induced SP suppressed by the treatment of CY alone were significantly restored by PL-treatment. These activities were higher by PL-M than by PL-W, These results indicated that PL was able to increase humoral immunity and to inhibit immunotoxicity induced by CY.