Respiratory manifestation in chickens revealed different bacteria during isolation from 155 chickens of different ages and localities suffered from respiratory sings. Rapid serum plate test for Mycoplasma Gallisepticum showed 36.8% positive samples. Bacteriological examination revealed, 359 isolates as follow, 85 E.coli, 62 Staph.aureus, 49 Proteus, 49 Klebseilla, 38 Salmonella, 37 Pseudomonas, 22 Shigella, 9 Pasteurella spp., 6 Serratia, and 2 Enterobacter. Serologically E.coli was O78, while Salmonella positive isolates were S.enteritidis, S.Kentucky, and S.anatum. Pathogenicity of isolated E.coli O78 & S. enteritidis(single or mixed infection) and its effect on mortality, body weight, some biochemical (total protein, albumin, uric acid, and creatinine) and immunological parameters(IgG, IgM, IgA) were studied.