Effects of royal jelly(RJ) on the immune system in normal and cyclophosphamide(CY)-treated mice were investigated. The results were as following: 1. Body weight, spleen weight, thymus weight, WBC, cell-mediated immunity (CMI, contact hypersensitivity to DNFB), humoral immunity (HI, Hemagglutinin-, Hemolysin-titer) were increased or decreased dependent on the day of administration of RJ in normal mice. But it showed no effect on liver weight and RBC. 2. Combined treatment with RJ in CY-treated mice on the day which RJ showed the increasing activities in normal mice inhibited the decrease of survival rate, body weight, spleen weight, WBC and CMI caused by CY, but no effect on the decrease of thymus weight and HI induced by CY.