Vaccination is an option for controlling Mycoplasma gallisepticum or M. synoviae when biosecurity measures fail to prevent the infection of poultry flocks with these mycoplasmas. Both killed vaccines (bacterins) and living vaccines are currently in commercial use. Bacterins usually contain an oil emulsion adjuvant and are administered by subcutaneous or intramuscular injection. They can reduce the decline in egg production associated with M. gallisepticum, although they do not prevent infection. Newer adjuvants, such as immune stimulating complexes, may provide effective immunity without the tissue lesions caused by oil emulsion adjuvants. Living M. gallisepticum vaccines include the F strain and attenuated strains ts-11 and 6/85. F strain is administered in drinking water or by aerosol. This strain reduces the decline in egg production and has been used to displace endemic strains in multiple-age flocks. The major disadvantage is the inherent virulence of F strain. Strain ts-11 is less virulent and less infectious than F strain and provides a somewhat weaker, but usually effective, long-term protective immunity, which is vaccine-dose dependent. This strain is administered by eye drop, persists in the chicken for long periods and stimulates a detectable although variable systemic antibody response. Strain ts-11 can be used safely in combination with respiratory virus vaccines. Strain 6/85 also stimulates a weaker protective immune response than F strain and is of low virulence and infectivity. This strain is administered by aerosol, appears not to persist in vaccinated birds and may fail to stimulate a detectable systemic antibody response. Strain MS-H is currently being evaluated as a live vaccine against M. synoviae in meat chicken breeder flocks and is often used in conjunction with strain ts-11.