This study was conducted during an outbreak of infections laiyngotracheitis (ILT) in three flocks

hyline breed of commercial layers with a total number of 20.000 housed in cage systems in Bal'a , East of Tulkarem . North Palestine .these flocks were previously vaccinated once at 15 weeks age with attenuated vaccine against ILT disease via cloacae . The clinical findings of the disease were : Respirator)' distress including gasping , coughing , gargling , marked dyspnea and expectorating of vigorously blood stained mucous and some layers showed existence of dried blood around the nostrils and lower beaks , unilateral or bilateral closed eyes , lacrimation and the egg production decreased 30% . The morbidity rate was high and the mortality rate reached 12% . The necropsy findings of dead birds showed mucoid tracheitis , laryngitis , sever hemorrhages in the trachea and the lumens were filled with mucus mixed with blood obstructing the trachea or larynx , exudates , caseous material and existence of blood casts along the entire length of the larynx and trachea . The disease was diagnosed by isolation of the ILT virus from the dead and sick birds tracheal suspension by culturing onto the chorioallantoic membrane (CAM ) of 10-12 day- old embryonated chicken eggs and identified by neutralization test using reference anti-ILT serum and quantitative detection of antibodies level in layers sera using ELT-LT EL1SA KIT - Jordan -Bioindustris Center ( JOVAC) . Histopathological study revealed characteristic intranuclear inclusion bodies in both experimentally infected cocks and tracheal section of sick layers .