Isogeneic (rat) and xenogeneic (swine) fetal liver fragments (FLF) transplantation into the omentum was performed for D-galactosamine (D-Gal)-induced acute and carbon tetrachloride (CCl4)-induced chronic hepatic failure in rats. The recipients that had iso or xeno FLF showed higher survival rates than the nontransplanted controls on a lethal dose (2.6 g/kg body weight) of D-Gal (survival rates: Iso 70%, Xeno 80%, and control 9.1%). On a sublethal dose (1.0 or 1.2 g/kg) of D-Gal, iso, or xeno FLF caused marked improvement of the values of GPT, GOT, and total bilirubin (T.Bil); at 72 h after D-Gal injection they went significantly lower than those of controls (Iso vs. control; p < 0.01, Xeno vs. control; p < 0.05). Histological examination of the livers revealed severe damage in controls, however, only a slight damage was found in iso or xeno FLF transplanted rats. Iso grafts were fairly well preserved in the omentum at 72 h posttransplants, however, xeno graft had almost changed into a necrotic tissue. CCl4 was administered subcutaneously for 14 wk to induce chronic hepatic failure and then iso FLF were transplanted 3 days after the last CCl4 injection. Iso FLF transplanted rats showed higher improvement of GPT and GOT values at 12 days posttransplants compared with controls (GPT p < 0.01, GOT p < 0.05), although histological improvement was not so remarkable in both group. Iso grafts formed nodules with many hepatocytes in the omentum 12 days posttransplant. The results indicate that iso or xeno FLF transplantation could be an alternative approach for incurable liver insufficiencies.