The duration and reversibility of the liver chalone-containing fraction on DNA synthesis in regenerating liver cells have been studied. It has been shown that administration of the [14C] chalone-containing fraction results in a rapid specific binding of the label to liver cells in which the label is retained for no less than 12 hours. The specific radioactivity of DNA in regenerating liver cells decreases 3-4-fold irrespective of the time of administration of the [14C] chalone-containing fraction. After repeated injections of the fraction under study, the hepatocytes loss their sensitivity towards this fraction. It is suggested that chalones exclude some part of the hepatocytes from the cell cycle which, in turn, leads to the asynchronous involvement in the cell cycle of a new hepatocyte population which is resistant to the chalone action.