Fifty day old Hubbard chicks were maintained and equally fed, at 2 weeks of age, they were divided into 4 groups each of 12 chicks according to the route of infection with M. gallisepticum. Blood and tissue samples were examined, it was found that the reisolation rate of mycoplasma was low (17%), macroscopic lesions were very mild, serological examinations were adopted, the number of positive sera and intensity of agglutination were high in the first week of infection, decreased gradually later on. Haemagglutination inhibition antibodies appeared later than the agglutinating antibodies, and increased later on and they were higher in air sac infection. Biochemical examination was performed, total serum protein increased in infected chicks and eletcrophoretic pattern using polyacrylamide gel electrophoresis showed that serum albumin decreased, total globulin O