Two hundred broiler breeders of 13 month old were examined from flock of 52000 hens . These breeders showing clinical signs in the form of rales , depression , low egg production and low hatchability . The rate of the embryos death reached 20% and dead embryos showing dwarfing and generalized odema. Mycoplasma gallisepticum (M.G.) was isolated from breeders and their dead chicken embryos using mycoplasma broth and Mycoplasma plate agar . The antibodies against M.G. were elevated in the affected breeders . The hyperplasia of their lining epithelial ,parabronchial lymphoid aggregation , in addition to pneumonia and thickened air sacs by leukocytes and lymphocytes could be seen . After treatment with tylosin for one-month failure of isolation of M.G. was noticed . Clinical signs , lesions , egg production and hatchability were improved . The antibodies were declined after 5 months of treatment . The tylosin was effective in the treatment and control of M.G. infection .