The changes in the phagocytic function of the reticuloendothelial (RE) system after 67% hepatectomy in rats were studied by use of 51Cr-endotoxin as the phagocytable material. The humoral opsonic index was significantly increased to approximately 1.2 to 1.8 (control, 1.0 +/- 0.2, mean +/- SD) until the fourteenth postoperative day. In contrast, the phagocytic index was decreased to 0.068 +/- 0.016 (control, 0.103 +/- 0.015) on the first day, then returned to the normal level on the second day. In this early postoperative period, uptake rates of 51Cr-endotoxin in the liver were remarkably decreased to about 50% to 70% of control, whereas those in the spleen and lung were increased two- to threefold of control. From the third to the fourteenth day, the phagocytic index was significantly increased compared with the preoperative level. During this period, the uptake rates in the liver and spleen were within the normal range. These results suggest that the increases in the opsonic index of 67% hepatectomized rats represent the homeostatic response for maintaining or stimulating RE system phagocytic function, and that high or normal phagocytic index, concomitant with the increase in the opsonic index, implies an enhanced or compensatory stage, respectively. The decrease in the phagocytic index despite the high opsonic index is assumed to represent a compromised stage of the RE system.