In the present study, reticuloendothelial system (RES) phagocytic function of rats with partial hepatectomy or experimentally induced liver cirrhosis was investigated by determining the phagocytic index, the opsonic index, and uptake rate in liver, spleen, and lung of a 51Cr-labeled endotoxin-injected rat. In both the partially hepatectomized and the cirrhotic rats, all three indicators varied markedly according to the elapsed period since liver injury. The changes in RES phagocytic function were classified into three different phases: compromised, compensatory, and enhanced. The compromised phase, consisting of a decrease in the phagocytic index, was observed during the first 24 hr after 67% hepatectomy and in advanced liver cirrhosis. This represented the failure of RES phagocytic function. The compensatory phase, in which the phagocytic index was maintained at nearly normal levels mainly by a compensatory enhancement in the opsonic index, was seen during the first to second postoperative day and in moderate liver cirrhosis. The enhanced phase, with a high phagocytic index, was observed from Day 4 to approximately Day 14 after surgery, and in the cases of mild liver damage. In the compromised and compensatory phases, the liver uptake rate was significantly decreased compared with the control. However, the uptake in the spleen and lung were markedly increased. In conclusion, the phagocytic function of the RES was significantly affected to a degree which changed with the extent of liver damage.