This paper is to report our method to extract a fairly purified lymphocyte chalone-like factor from pig spleen and our observation on its properties and actions.An acetone powder was prepared from pig spleen and it was extracted with redistilled water. After ultracentrifugation, the supernatant was ultrafiltrated and the filtrate was called fraction B. Fraction B was further fractionated by chromato-graphy on Sephadex G-25 Column. 7 peaks were obtained(SP1-SP7)and the strongest biological activity was found in the fraction SP2. It is the lymphocyte chalone-like factor and its molecular weight is 1400.The actions of this lymphocyte chalone was not affected in the condition of incubation when the calf serum of the culture medium was subsituted with human calf serum or different doses of insulin were added to the medium.[3H]-TdR and [3H]-Urd incorporation experiments showed that the inhibition of RNA synthesis of this factor occurred prior to the inhibition of the scheduled DNA synthesis. It is suggested that the mechanism of this lymphocyte chalone actions is likely to inhibit the activation process of some genes, then inhibit the DNA synthesis and block the cells from G1 phase to S phase.