Immunoblotting analysis revealed immunodominant properties and the most intensive cross-reactivity of 39-44 kDa, 64-70 kDa and 77-80 kDa proteins of mycoplasma strains and isolates tested. Non-pathogenic M. gallinarum species isolates did not express antigenic heterogeneity. Protein polymorphism of pathogenic M. synoviae species isolates were determined by both the differences between intensity of expression of 77-88 kDa protein and the differences in size of 38-46 kDa proteins. Soluble antigen fraction and polyvalent sera testing by immunoblotting analysis enables to determine morphological or antigenic heterogeneity of strains but fails to reveal species-specific features.