Isolated cryopreserved human liver cells, attached to collagen-coated microcarriers, were injected intraperitoneally into mutant rat recipients genetically deficient in either bilirubin uridine diphosphoglucuronosyltransferase activity (Gunn rats) or albumin synthesis (Nagase analbuminemic rats). One group of the recipient Gunn and analbuminemic rats were made genetically immunodeficient by interbreeding with athymic rats with inherited T-cell deficiency. Injected microcarriers and cells formed aggregates on the surface of the pancreas. There was no morphologic evidence of rejection in athymic recipients, whereas immunocompetent recipients demonstrated rejection within 5 days of transplantation. Athymic-Gunn rat recipients demonstrated excretion of bilirubin glucuronides in bile for 30 days and reduction in their serum bilirubin levels. In recipient athymic-analbuminemic rats, plasma albumin levels increased from pretransplantation levels of 0.025-0.05 mg/ml to 3.9-4.8 mg/ml 8 days posttransplantation and remained nearly at that level throughout the 30 days of the study. A method of harvesting, attaching to microcarriers, cryopreserving, and in vivo testing of human hepatocytes with prolonged survival and function in athymic-Gunn and athymic-analbuminemic hybrid rats is reported. These rats are excellent animal models for testing xenograft function.