Inactivated oil emulsion Newcastle disease (ND) and infectious bronchitis (IB) vaccines were prepared and evaluated in both monovalent and bivalent form. By using such vaccines as secondary boosters an enhanced immune response was produced for up to 10 weeks, at which time there was a high degree of resistance to challenge with IB virus in the bivalent vaccine group. This secondary response was most effective when preceded by a live combined ND/IB primary vaccine, which in itself did not stimulate a satisfactory antibody response. Revaccination with a bivalent inactivated vaccine containing the adjuvant BRL 5907 produced a secondary response significantly better than in the groups that received the bivalent vaccine only. When the mono- and bivalent inactivated vaccines were used as primary vaccines a poor immune response was illicited. However, revaccination with a bivalent inactivated vaccine significantly improved the response. An interesting observation was that when ND virus and IB virus were grown in the same embryonated eggs there was an increase in IB virus yield. When inactivated, and emulsified as a bivalent vaccine an enhanced immune response to IB virus was induced in the vaccinated chickens.