A sequential analysis of liver allograft rejection in sensitized rats using immunopathological and ultrastructural microscopy is described. Lewis rats were primed with four ACI skin grafts and challenged with an arterialized ACI orthotopic liver allograft 14 to 17 weeks later. The sensitization resulted in a mix of IgG and IgM lymphocytotoxic antibodies at a titer of 1:512 at the time of transplantation. Specificity analysis of pretransplant immune sera revealed a predominance of IgG anti-class I major histocompatibility complex (RT1) antibodies with a minor IgG fraction showing apparent endothelial cell specificity (non-RT1). This level of sensitization was associated with accelerated graft failure in 3 to 5 days from mixed humoral and cellular rejection. Sequential analysis of serial posttransplant graft biopsies revealed diffuse vascular IgG deposition and platelet thrombi in portal veins and periportal sinusoids within 3 minutes after reperfusion. This was followed by endothelial cell hypertrophy and vacuolization, periportal hepatocyte necrosis, arterial spasm, focal large bile duct necrosis, and hilar mast cell infiltration and degranulation. However, the liver allografts did not fail precipitously and hyperacute rejection was not seen. Kupffer cell phagocytosis of the sinusoidal platelets began as early as 30 minutes posttransplant and by 24 hours, the platelet thrombi had decreased. Cholangioles appeared focally at the edge of the limiting plates by 2 to 3 days, apparently in response to earlier periportal hepatocyte damage. A mononuclear portal and perivenular infiltrate became evident at 3 days, and graft failure was attributed to both antibody and cell-mediated rejection (Furuya et al: Preformed lymphocytotoxic antibodies: Hepatology 1992, 16: 1415-1422). The model described resembles observations in crossmatch positive human liver allograft recipients. The mechanisms of hepatic graft resistance to antibody mediated rejection and the possible long term consequences of early damage to the biliary tree are discussed.