30 One day old Mycoplasma free broiler were divided into 6 groups (grs) (each of 5 chicken). The 1^st gr. used as control without any treatment, 2^nd gr. was infected with Mycoplasma gallisepticum (M.G.) S6 via the thoracic air sac on the 16^th day of age and left till the end of the experiment (as infected control gr.), 3^rd gr. was infected on the 16^th day then passively immunized by I/P injection of egg yolk antibodies (IgY), 4^th gr. was passively immunized I/P by (IgY) and infected with M.G., then passively immunized, 5^th gr. was vaccinated with the living vaccine (F-strain) of M.G. then infected with M.G.S6 and the 6^th gr. was  orally given the therapeutic dose of enrofloxacin in the drinking water for the 1^st 5 days of age, then infected with M.G.S6 and the chickens were given another dose of enrofloxacin from 20^th -25^th day of age.      Blood samples were collected from each bird in grs. (1, 4, 5 and 6) at the end of the 1^st, 2^nd, 3^rd and 5^th weeks of age and from grs. (2 and 3) at the end of the 3^rd and 5^th weeks for serological, hematological and biochemical tests. The lungs, trachea and air sacs were collected for the isolation of M.G. The infected birds gr. (2) revealed Leukocytosis, heterophilia and monocytosis in the 3^rd and 5^th weeks of age. There was  a highly signififent increase in the AST, ALT, AP, Total proteins, globulin and uric acid. The serum albumin was decreased, while the creatinine not significant changed. The ELISA showed positive values as early as the 5^th day post infection (PI) till the end of the experiment. The result of serum plate agglutination test (SPA) illustrated 3/5 and 5/5 at the end of the 3^rd and 5^th weeks of the age respectively. The Pulmonary tissue was congested and showed focal pneumonic areas. The passively immunized birds of gr. (4), before infection, showed non-significant change in leukogram but there are a significant increase in the total protein and globulin at the end of the 2^nd week. There was a positive result of the EILSA due to the injected (Ig). The SPA was 0/5 and 0/5 in the 1^st and 2^nd weeks of age respectively. The passively immunized and infected chickens grs. (3&4) showed similar results to gr. (2) at the end of the 3^rd week. All the parameters returned to nearly the normal value in the 5^th week of age except for a significant increase in the serum total protein and globulin. The ELISA showed a positive result in the 3^rd and 5^th weeks of age. The SPA showed 3/5 and 2/5in grs. (3&4) respectively in the 3^rd week and 1/5 in the 5^th week of age in both grs. The actively immunized birds of gr. (5) showed leukocytosis, heterophilia, lymphocytosis and monocytosis at the end of the 1^st, 2^nd, 3^rd and 5^th weeks of age. All the parameters were not significantly changed except the total proteins and globulin which showed highly significant increase. The ELISA was gradually increased from the 2^nd till the 5^th week of age. The SPA showed 0/5, 1/5, 4/5 and 5/5in the 1^st, 2^nd, 3^rd and 5^th weeks of age respectively. The enrofloxacin treated birds gr. (6) before infection especially at the end of the 1^st week of age, showed leukopenia, heterophilia, lymphopenia and a significant increase in AST, ALT, AP, uric acid and creatinine. Meanwhile, the serum total proteins, albumin and globulin were non-significantly changed. The ELISA showed non-significant change. The SPA is 0/5in the 1^st & 2^nd weeks of age. The enrofloxacin treated birds gr. (6) after infection and at the end of the 3^rd week of age showed similar hematological finding to gr. (2) at the end of the 5^th week of age. All parameters nearly regained to their normal values except for a significant increase in the total proteins and globulin. The ELISA showed a significant increase. The SPA were 3/5 and 1/5 at the end of the 3^rd & 5^th weeks respectively. The chickens which were sacrificed at the end of the 3^rd week, revealed focal hemorrhage in the lung and serofibrinous exudates. It is recommended to vaccinate the chickens with F-strain living vaccine of M.G.