Yolk was separated from eggs laid by healthy hens which were inoculated with infectious bursal disease virus (IBDV) and Newcastle disease virus (NDV) several times, and a combined hyperimmune yolk-fluid against IBDV and NDV was obtained. Its haemagglutination-inhibiting (HI) antibodies to NDV as well as neutralizing and precipitating antibodies to IBDV reached a high level. The results of experimental and field triab showed that inoculation of this combined antibody product could , within a short time, allow chickens to withstand IBDV and NDV challenge. When injected into chickens early in an outbreak of these diseases, it showed marked therapeutic efficacy.