Infection by hepatitis B (HBV) and/or delta virus (HDV), is the most frequent acquired pathology in patients affected by end-stage hepatic disease, candidates for liver transplant. To reduce the risk of virus reactivation after surgery, we used alpha Interferon (IFN) therapy in patients who were HBV-DNA and/or HDV-RNA positives before transplant. Our protocol included alpha IFN at low dosage associated to a thymic hormone that seems to have a synergistic activity with IFN. We have evaluated in four patients, affected by post hepatitic end-stage liver disease, the outcome of HBV and HDV markers in relation to immunological response during treatment. Our interest has been focused on monocyte and natural killer cytotoxic activity. The data show that all patients, before starting therapy, had evidence of active phase viral replication. They also displayed low values of the immunological parameters tested. The study of viral markers showed decrease of HBV and HDV in all patients. The relation between viral markers and natural killer and monocyte cytotoxicity was very interesting; during the treatment we observed a marked increase of both activities. At the same time no relevant modifications in the other immunological parameters tested were found.