The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) plays an important role in the pathogenesis of Newcastle disease (ND). Recombinant HN (rHN) protein, produced either by direct injection of recombinant viruses containing HN gene or baculovirus expression systems, has been used to elicit immunity against NDV in chickens. In the present study, a 60.4-kDa rHN was expressed by a prokaryotic expression system and formulated into ND vaccines. Inclusion of rHN (10 microg/ml) into conventional, inactivated ND vaccines significantly (P < 0.05) increased the titer of serum hemagglutination-inhibition Ab in specific-pathogen-free or commercial chickens. Furthermore, when the rHN protein was formulated into ND+IC (infectious coryza) bivalent or ND+IC+FC (fowl cholera) multivalent vaccines, the protection rate of immunized chickens increased from approximately 80%-90% to 100% after being challenged by a velogenic strain of NDV. Our data indicated that inclusion of rHN protein produced by an economical prokaryotic expression system could enhance the immunogenicity of traditional and multivalent inactivated ND vaccines. This approach may be adapted to improve the efficacy of ND vaccines currently used in the poultry industry.