Four broiler houses in Sharkia governorate were examined to studythe microclimatic condition, occurrence of airborne bacteria, their effectson broiler performance and occurrence of pathogenic bacteria in air indifferent seasons.1- Microclimatic conditions in broiler houses:determined to danofloxacin and other five members of commonly usedquinolones.The efficacy of therapeutic doses of danofloxacin in chicks experimentally infected with Mycoplasma, E. coli and mixed infection of . both microorganisms comapred with tylosin, oxolinic acid and lincospectinrespectively was investigated.Special emphasis was drawn to study the drugs induced side effects that could be reflected on some liver and kidney functions and histopathological changes involving, trachea, lungs, liver and kidneys.A- Antibacterial activity in-vitro:The present study demonstrated that MICs of novel fluoroquinolones(danofloxacin, enrofloxacin and norfloxacin) were much lower than that of older quinolones (flumequine, oxolinic acid and nalidixic acid) against tested avian pathogens namely Salmonella, Arizona, Proteus, Pseudomonas, Staph.aureus, Staph. citrus, Strept. faecalis and E. coli.The sensitivity test is of a great value in recommendation of theeffective antibacterial agents. Our results revealed that the tested isolates