الفهرس 
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Abstract
The prevalence of infectious coryza among different chicken farms at Assiut, Elmenia, Sohag, Kena and EI-Wady-EI-Gadeed Governorates was studied. Seven hundreds and thirty alive and freshly dead chickens of different ages and breeds were subjected to clinical, post-mortem and bacteriological examination. Isolated Haemophilus organisms were identified morhologically, biologically, biochemically, and serologically as Haemophilus paragallinarum serotye 2. Infected birds showed signs of nasal discharge, swelling of the infraorbital sinus, facial swelling, retardation of growth in growing birds and DROP of egg production in laying birds. The course of the disease in non complicated cases was 7-12 days but in complicated coryza •the signs were sever and the course was extended up to 3-4 weeks. Isolated Haemophilus organisms were pathogenic to chicken embryos causing death within 24-48 hours with congestion of the dead embryos and its membranes. In chickens the pathogenicity of the isolates depended upon the route of infection, all 15 isolates were pathogenic to chickens by intrasinus inoculation but only II isolates were pathogenic by intranasal inoculation Flocks showing atypical signs, lesions and course were complicated with Mycoplasma gallisepticum and E-coli. Experimental mixed infections by Haemophilus organisms and Mycoplasma gallisepticum or E-coli resulted in prolonged course, signs and lesions as those observed in the field. It was found that monovalent bacterins prepared either from serotype 1 or serotype 2 protect the vaccinated birds against challenge with homologous. strains but not lIIf:ainst heterologous strains which indicated that their is no cross-protection between the two serotypes. Bivalent bacterin induced good protection for chickens against challenge by both serotypes , It was concluded that their is a correlation between agglutination titers and hemagglutinatin inhibition titers on one hand and protection against chall enge - exposure on the other hand.