The strain of infectious laryngotracheitis (ILT virus utilised for these vaccine-development studies was isolated from an outbreak of the disease in Hungary. Inactivating agents tested included beta-propio-lactone, methyleneimine, ethyleneimine EI and formalin. ILT virus preparations inactivated with EI appeared to be the most antigenic when inoculated into chickens in the absence of adjuvant. For formulation of a trial oil-emulsion ILT vaccine for experimentation, ILT virus inactivated with 1200 mug/ml of EI was mixed with an equal volume of 50% incomplete Freund's adjuvant and then homogenised. Experimental vaccination of chickens with inactivated ILT vaccine elicited satisfactory serological response and protection to challenge both under laboratory and field conditions. Although only minor increases in antibody titres could be achieved by repeated vaccination with inactivated ILT vaccine at 2 to 5 weeks after primary vaccination at 10-weeks-old with attenuated or inactivated ILT vaccines, the protection conferred by vaccination with a single dose of the trial inactivated ILT vaccine appeared to be effective for at least 12 months. Hence, there would appear to be potential for inactivated-ILT vaccines in control of ILT, especially on sites that are regularly affected by this infection.