Rather the term "reference" immunogenic composition refers to an immunogenic composition obtained by the cultivation of mycoplasma bacteria in a cell free culturing system supplemented with serum using the same antigen. The cultivation of mycoplasma bacteria in a cell free culturing system supplemented with serum is well known to the person skilled in the art. The serum origin and the serum concentration are dependent on the mycoplasma bacteria to be cultivated and the yields of mycoplasma bacteria to be obtained. M. hyorhinis,
M. hyopneumoniae and M. hyosynoviae are generally cultivated in a cell free culturing system supplemented with 10-20% (v/V porcine serum and 5-10% (v/v) yeast extract to obtain high yields. However, it is to be understood that the serum concentrations can be varied, the yeast extract can be deleted and the origin of the serum can be changed, exemplary to fetal calf serum or the alike.