Liver biopsies obtained from 10 liver transplant patients for whom a cytomegalovirus (CMV) hepatitis was suspected on the basis of clinical and biological abnormalities have been studied by an immunohistological method using the monoclonal antibody E-13 directed against an early viral antigen. Biopsies were performed between the 30th and the 77th day after transplantation. In 7 of the 10 cases, CMV hepatitis was diagnosed by histological examination because of the association of lobular cytolysis, inflammatory infiltrate with neutrophils and intranuclear inclusions. Ten cases were positive with the E-13 antibody including the 3 cases in which histological examination was not conclusive. In all cases, positive results gave a nuclear staining pattern in hepatocytes (63%) or endothelial cells (40%), independently of the presence of inclusions. Rapid diagnosis of CMV hepatitis is of significant importance since anti-viral drugs, such as DHPG (Ganciclovir), are now efficient. The immunohistochemical method with E-13 could permit easy and rapid detection of CMV in liver specimens.