Renewed interest in the transplantation of isolated hepatocytes into the liver as a potential therapy for liver disease has stimulated the development of methods for the identification of donor cells within the recipient organ. We describe a method for cellular tagging and in vivo identification of intraportally transplanted hepatocytes using an intracellular fluorescent dye, 5(6)-carboxyfluorescein diacetate, succinimidyl-ester (CFSE). Rat and porcine hepatocytes were isolated and labelled with CFSE. The optimal conditions for labelling consisted of a buffered saline suspension of hepatocytes (5 x 10(6) cells/mL) in 20.0 microM CFSE incubated for 15 min at 37 degrees C. In vitro, labelled hepatocytes were cultured either on fibronectin-coated chamber slides or in culture flasks. Cultures were evaluated in situ by fluorescence photomicrography or by fluorescence-activated cell sorting (FACS) after cell detachment. Cell viability was assessed serially and cultured, labelled hepatocytes retained the dye for up to 3 wk (last day of study). CFSE did not effect hepatocyte viability and there was no evidence of intercellular diffusion of the dye. In vivo, syngeneic Lewis rats underwent selective portal vein infusion of freshly isolated, labelled hepatocytes (2.0 x 10(7) cells/2.0 mL saline/animal) into the posterior liver lobes. All recipients were sacrificed 48 h and 96 h later and their livers examined. Transplanted hepatocytes were identified by fluorescence microscopy in tissue sections and by FACS following collagenase digestion of the liver tissue. CFSE persisted in a population of viable, engrafted hepatocytes. FACS analysis demonstrated that 9 +/- 3% of the hepatocytes in the posterior liver lobes were labelled 48 and 96 h after transplantation. At 96 h following transplantation, multiple engrafted hepatocytes could be observed by fluorescence microscopy around the central veins. CFSE labelling allows for both in vitro identification and in vivo localization of donor hepatocytes. Furthermore, it appears to be more stable and specific for labelling hepatocytes than other tested dyes (especially DiI).