This study was carried out to investigate the possible bacterial causes affecting the
productivity of the breeder hens. The hens under test were positive for Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS) and negative for Salmonella gallinarum-pullorum (S. g-p) as tested by serum agglutination test. Ovaries with lesions were bacteriologically and serologically proven to have 12 bacterial isolates including three un-typed E. coli, one O11, and one O78; three Staphylococcus aureus; one S.g-p; two Proteus and one Pseudomonas aeruginosa (P. aeruginosa)
isolates. The antibiogram of the two identified E. coli strains, S. g-p as well as P. aeruginosa showed variable sensitivity. The tested organisms were very sensitive to colistin and enrofloxacin. The selected E. coli, S.g-p and P. aeruginosa strains proved to be pathogenic to 3-day-old chicks, with induction of clinical signs, mortalities, post mortem lesions and adverse effect on the body weight gain. On the other hand, E. coli O78 was more pathogenic than O 11. The use of the abovementioned drugs for controlling of those infections in chicks was effective as indicated by the results of the in vitro test. It is important to carry out bacteriological examination to the breeder flocks in order to investigate the bacterial affections with estimation of changes in their sensitivity to the used antibiotics.