BACKGROUND Inclusion of positively charged polymers such as protamine in adenovector formulations has been reported to improve the efficiency of adenovirus-mediated gene transfer in vitro and in vivo. On the other hand, corticosteroids are known to inhibit inflammation and thus might be useful in minimizing vector-related toxicity. In this study, we evaluated the combined effect of protamine sulfate and hydrocortisone on the efficiency of adenovirus-mediated gene transfer in vitro and in vivo. METHODS Protamine and hydrocortisone at different concentrations were added to adenovector formulations. In vitro transgene expression with or without inclusion of protamine and hydrocortisone was evaluated in the breast cancer cell lines MDA-MB-231 and MCF7 and the lung cancer cell lines A549 and H460. In vivo transgene expression in the mouse lung was determined after aerosolized vector delivery. RESULTS The combination of 2 micro g/ml protamine and 125 ng/ml hydrocortisone significantly increased transgene expression in vitro in all the cell lines tested. Protamine is only effective when it is added to cells before or together with adenovectors, whereas hydrocortisone is effective when it is added to cells before, together with, or after adenovectors. Inclusion of protamine and hydrocortisone also augmented apoptosis induction caused by adenovectors expressing proapoptotic genes in cancer cells. Moreover, protamine and hydrocortisone dramatically enhanced transgene expression in the mouse lung after aerosolized vector delivery. CONCLUSIONS Inclusion of protamine and hydrocortisone in adenovector formulations can improve adenovector-mediated gene expression and may be useful for clinical applications of current adenovirus-mediated gene therapy.