The present work was carried out to develop a simple and effective method for differentiating infected from vaccinated chickens to improve the control of infectious diseases. The differentiation of vaccinated and infected birds is based on addition of a suitable exogenous marker to the vaccine either during its preparation or mixed with it just before vaccination. This study has evaluated that the use of a tetanus toxoid (TT) marker in chickens vaccinated with Newcastle disease virus (NDV) and avian influenza (AI) vaccines eliciting a strong, protective antibody response and enabling serological discrimination between vaccinated and virus-infected chickens. All chicken groups were seronegative for TT-specific anti- bodies without vaccination, however post vaccination with three different doses, they elicited high levels of TT-specific antibodies that persisted all over the experiment regardless the used dose. Incorporation of TT with inactivated NDV vaccine during its manufacture elicited strong TT and Newcastle-specific antibody responses. Furthermore, vaccination with combined doses com- posed of avian influenza vaccine and TT induced high levels of antibodies to both antigens. There was no detectable interference by incorporation of TT in both inactivated NDV and AI vaccines for the viral antigens or TT-seroconversion. Thus TT is recommended as a suitable exogenous marker for avian vaccines. Moreover, testing the antibody response to the marker would confirm approved vaccine use and the antibody responses to the viral antigen would determine levels adequate for protection or indicate recent infection.