A factor has been purified from rabbit liver which decreases the incorporation of 3H-thymidine in DNA of regenerating rat liver slices. This effect is due mostly to an inhibition of DNA synthesis from the deoxynucleoside triphosphates. The purified rabbit liver factor thus interferes with the liver cell division cycle. This inhibitor of DNA synthesis is specific for liver cells; it is not toxic for cultured hepatocytes and its action on DNA synthesis is reversible: at low dose, the inhibition of DNA synthesis in regenerating liver slices is transitory. The purified rabbit liver inhibitor is thus a chalone. The purified rabbit liver chalone ultrafiltrates through 1.2 nm pores, is destroyed by trypsine and pronase, carries a negative charge at pH 8.8 and a positive charge at pH 4.6; it is thermostable and likely a small peptide. It also inhibits RNA and protein synthesis in regenerating liver slices. The inhibition of protein synthesis is immediately maximal then decreases with time, while the maximum inhibition of DNA and RNA synthesis appears after a delay. When a low dose of chalone is used (0.2 unit per 5 ml), the inhibition of DNA and RNA synthesis disappears after some time: this is not due to a destruction of the chalone, but to a loss of sensitivity of the slices incubated in Hanks solution. The inhibitor content of liver cells, normal or malignant, seems inversely correlated with their state of growth. It is much lower in the liver of a young animal or in regenerating liver than in adult liver. Hepatomas produced by feeding DAB contain three times less inhibitor than the normal liver. The purified liver chalone is 5-10 times less active on the incorporation of 3H-thymidine in DNA of DAB hepatoma slices than in DNA of regenerating liver slices. It has no apparent action on adult liver slices; this might be due to the fact that 3H-thymidine incorporation into DNA of the adult organ depends, for the greater part, on other processes than DNA replication in hepatocytes.