To evaluate the pathogenesis of live infectious laryngotracheitis vaccine, virus, virus and E.coli were inoculated in 4-weeks old intraorbitally and intraorally. Their pathology and serology were studied. The results were as follows. 1. thicks Inoculated with ILT vaccine alone showed no clinical sign. 2. Some of chicks inoculated with ILT vaccine followed by E.coli after 1day showed nasal discharge and cough. 3. Some of chicks inoculated with E. coli followed by ILT vaccine after 1 day and inoculated ILT vaccine and E.coli concurrently, showed nasal discharge. 4. Serum neutralization titers of chicks inoculated with ILT vaccine increased 3 weeks after Inoculation, peaked at 7 to 8 weeks and decreased. 5. Antibiotic sensitivity of E.coli were higher in Enrofloxacin (Baytril) and chloramphenical.