We examined platelet adhesion in thirty human donor livers to determine if the degree of platelet adhesion could predict outcome of transplantation. Wedge liver biopsies were taken at the start of the donor operation (biopsy 1) and 1 hr after reperfusion in the recipient (biopsy 2). Biopsies were stained with a monoclonal antibody against platelet glycoprotein Ib and graded for platelet adhesion. Hematoxylin and eosin-stained sections were examined for polymorphonuclear leukocyte adhesion and necrosis. Platelet adhesion was much more frequent and extensive than expected in biopsy 1. Nine of 30 biopsies showed moderate or high-grade platelet adhesion. Thus in this study endothelial cell damage was present in about one-third of donors before the donor operation. The injury was not detectable by routine microscopic or clinical examination or biochemical tests. The degree of platelet adhesion in biopsy 1 predicted development of PMN adhesion and necrosis in biopsy 2 and postoperative transaminase concentrations and prothrombin times in recipients. During preservation and implantation some livers converted from low to either moderate or high grades of platelet adhesion. The grade of platelet adhesion in biopsy 2 predicted postoperative outcome as measured by transaminase and PT levels. Patients whose platelet grade converted to a higher level during preservation and implantation did not do as well as patients who remained at a low adhesion grade. These findings strongly suggest that the degree of platelet adhesion is an important determinant in assessing outcome and may provide a means of measuring the status of liver allografts prior to transplantation.