The distribution of major histocompatibility complex (MHC) class 1 antigens was studied in the liver after transplantation by immunoperoxidase staining for beta-2-microglobulin (beta 2m), a subunit of the class 1 antigen system. Paraffin wax sections were examined from 25 "time zero" biopsy specimens, taken immediately after insertion of the graft, and 87 biopsy specimens taken after transplantation in seven diagnostic categories: acute cellular rejection (n = 22); resolving acute rejection (n = 8); chronic rejection (n = 22); pure cholestasis (n = 14); ischaemia/infarction (n = 5); biliary obstruction (n = 8); massive haemorrhagic necrosis (n = 8). Staining was graded semiquantitatively on a scale of 0-3+ in bile ducts, hepatocytes, sinusoidal lining cells and vascular endothelium. Using the "time zero" biopsy specimens as a baseline for comparison, increased expression of beta 2m was seen in bile ducts, hepatocytes, and endothelial cells after transplantation. These changes were most pronounced in cases of rejection but also occurred in other graft conditions. The degree of hepatocyte and endothelial staining was significantly higher in cases of rejection and massive haemorrhagic necrosis than in the other categories. These findings may have implications for the pathogenesis and diagnosis of rejection of the transplanted liver.