I can't seem to get rid of mg in my chickens
I'm sorry you are having trouble.
Exactly what do you mean by you "can't get rid of it"?
Do you mean that your chickens still have symptoms?
What type of antibiotics have you been using and at what dosage?
What symptoms are you seeing?
What are your current and future chicken keeping goals?
@SunHwaKwon has asked a very good question - how was it diagnosed - did you have testing/necropsy, etc.?
MG (Mycoplasma Gallisepticum) is a contagious respiratory disease. You never really "get rid of it". Even if your chickens are not currently showing symptoms, they are still carriers of the disease for life. Any new birds that you add to your current flock, will be considered carriers, even if they never show any symptoms. MG is transmitted both horizontally (bird to bird, through dust/dander, feathers, mucous, etc.) and vertically (transovarian/through the egg). Unless you cull all, clean and wait for a period of time, you can't get rid of it (this does not always work - you may still end of with it).
Antibiotics will only treat secondary infections that accompany respiratory illness, they will not cure the disease. Birds can still become symptomatic periodically throughout their lives during times of stress or if they become ill/weakened by something else.
http://ucanr.edu/sites/poultry/files/201395.pdf
http://www.thepoultrysite.com/disea...tion-mg-chronic-respiratory-disease-chickens/