Introduction: Using a partial liver graft obtained from a living donor takes advantages of the unique capability of the liver to regenerate. This study aims to validate the immunohistochemical (IHC) assay to quantify liver regeneration, to investigate the kinetics of regeneration after transplantation and to study the negative feedback mechanism between immune activation and liver regeneration. Materials and Methods: Lewis rats were used as donors for the 30% partial liver grafts (POLT) and as recipients, as well as for 70% and 90% liver resection (PH). Kinetics of regeneration was compared after 70%PH and 30%POLT. The influence of prolonged ischemia was assessed by comparing the hepatocyte proliferation rate in liver grafts subjected to either 1h, 3h or 5h of cold ischemia. The influence of immune activation on liver regeneration was studied by subjecting the animals to hepatitis B vaccination prior 70%PH in comparison to the untreated hepatectomized rats. The effect of inhibition of immune activation was investigated by using either drugs suppressing IL-2 production (Calcineurin inhibitors CsA and FK506) or inhibiting T-cell proliferation (MMF and RAD). BrdU 50mg/kg was injected intravenously 1h prior to sacrifice. Paraffin embedded liver samples were used for IHC detection of the incorporated halogenated nucleotide. Results: Comparison between the conventional and computer-assisted quantification of the hepatocyte nuclei revealed a strong correlation between the two methods (r=0.868). Intra- and Inter-assay variation were low (CV<10%), demonstrating the high precision of the assay. Regeneration following 70%PH peaked at day 1 postoperatively. Transplantation delayed the onset of liver regeneration by 24 hours. Prolonged ischemia reduced the maximal LI. Immune activation prior to 70%PH did not alter the kinetics of regeneration, but reduced the maximal proliferation rate, possibly related to the increased IL-2 secretion in response to vaccination. Inhibition of IL -2 production by calcineurin inhibitor tended to increase the proliferation rate, whereas the use of antiproliferative drugs blocked liver regeneration almost completely.Conclusions: Transplantation as well as prolonged ischemia postponed and impaired regeneration. The hypothesis regarding the immunological regulation of liver regeneration was supported, as immune activation inhibited regeneration and inhibition of IL-2 production by CsA enhanced the regeneration rate.