De novo immune hepatitis (DNIH) is a form of late graft dysfunction after liver transplantation. The fine mechanisms leading to the development of DNIH are not known, and whether this hepatitis is a form of rejection or a result of an auto/alloimmune injury has not been established. In our patients, DNIH was always preceded by the production of donor-specific antibodies against the glutathione S-transferase T1 (GSTT1) enzyme because of a genetic mismatch in which the donors carried the wild-type gene and the recipients displayed the null genotype. Complement component 4d (C4d) immunopositivity in 12 paraffin-embedded liver biopsy samples from 8 patients diagnosed with DNIH associated with anti-GSTT1 antibodies was retrospectively evaluated. Six patients with a diagnosis of chronic rejection (CR) and 7 patients with hepatitis C virus recurrence were included as control groups. Among the patients with DNIH, 7 showed C4d-positive immunostaining localized in the portal tracts, whereas in the tested biopsy samples of the 2 control groups, this staining pattern was absent. Four biopsy samples of the CR group showed C4d-positive sinusoidal staining. This study confirms the activation of the complement pathway in the presence of donor-specific antibodies, which was shown by the deposition of C4d elements in liver biopsy samples of patients with DNIH. The use of C4d as a marker of antibody-mediated rejection in liver allografts in the presence of antidonor antibodies is discussed, and it may contribute to improved differential diagnoses based on biopsy findings. Liver Transpl 17:779-788, 2011. © 2011 AASLD.