Isolated hepatocytes, prepared from 5 day old rats, from regenerating livers of from livers after poisoning with carbon tetrachloride, are less sensitive to phalloidin in vitro than hepatocytes from untreated adult controls. The time course of the reduced susceptibility to phalloidin was compared with the ability of hepatocytes to take up bile acids under various conditions. SDS-electrophoresis of cell lysates gave no evidence for decreased levels of actin in cells with reduced sensitivity to phalloidin. In contrast, there was a good relationship between the active uptake of bile acids and the sensitivity of hepatocytes to phalloidin. The decreased response of hepatocytes from baby rats, from regenerating livers or from poisoned livers to phalloidin is more probably related to differences in phalloidin uptake than to a reduced endowment with microfilamentous structures.