The activity of colony stimulating factor ( CSF) in the conditioned medium ( CM ) was studied with combination method of 3H-TdR incorporation assay and agar colony assay. These two assays were demonstrated to be replaceable each other. The mice were administered with nontoxic monophosphoryl lipid A(MPLA) which was derived from a Re mutant of Salmonnella Minnesota Re595. The results showed a significant elevation of CSF in the serum and reaching the top at 12th h and returning to normal by 24th h. There is a significant dose-resoonse relationship. The CSF was induced with accompany of formation of colony inhibiting factor (GIF), some of which were heat sensitive factors. It is suggested that the MPLA may be a potent CSF-inducer.