The activities and properties of two inhibitors of cell proliferation, which are present in the supernatant of rat liver, have been studied in L(929) fibloblast systems. 1) One factor, which had arginase activity, completely inhibited cell proliferation by depletion of arginine in the culture medium. 2) The other factor was stable to Trypsin-, RNase and Heat-treatment, and primarily inhibited protein synthesis. Inhibition of DNA synthesis was thought to be a secondary effect. 3) Whether the second factor changes the biological activity in regenerating liver is now under investigation.