Restoration of liver function in Gunn rats without immunosuppression using transplanted microencapsulated hepatocytes. Microencapsulation of cells within synthetic semipermeable membranes is a novel technique that enables the transplantation of cell cultures without the need for immunosuppression. We have previously shown that transplanted isolated encapsulated hepatocytes can provide sufficient short-term metabolic support to improve the survival of animals with galactosamine-induced fulminant hepatic failure. Here we have demonstrated the feasibility of isolated encapsulated hepatocyte transplantation in providing long-term metabolic liver support in Gunn rats. Gunn rats have a congenital inability to conjugate bilirubin and thus exhibit lifelong hyperbilirubinemia. We studied the feasibility of isolated encapsulated hepatocyte transplantation in restoring this specific liver function. Free hepatocytes, isolated from male Wistar rats, were microencapsulated with collagen within a trilayered sodium alginate-poly-L-lysine-sodium alginate membrane using techniques developed in our laboratory. A total of 45 Gunn rats underwent intraperitoneal transplantation with free hepatocytes (5 x 10(7], isolated encapsulated hepatocytes (5 x 10(7], control (empty) microcapsules or no transplant (untreated controls). Serum bilirubin levels were monitored daily for 10 days after transplantation, and subsequent weekly samples were obtained for up to 1 mo. Microcapsules were studied by light and electron microscopy 1 mo after transplantation. During the first week after transplantation, the mean maximum reduction in serum bilirubin levels for the isolated encapsulated hepatocytes, free hepatocytes and control microcapsule transplanted groups was 45.7%, 18.6% and 14.3%, respectively. For up to 1 mo thereafter the mean reduction in serum bilirubin levels in these respective groups was 34.8%, 13.5% and 3.3%. 