The strains of swine mycoplasma (Tab. 1) were grown aerobically in Whittlestone's Medium containing swine serum for 4-5 days at 37 degrees C. After centrifugation and washing they were freeze-dried and extracted with a phenol-acetic acid-water solution (4:2:1) (1 ml/5 mg mycoplasma dru weight) at 4 degrees C over 48 h. After centrifugation 2 volumes of the supernatant were mixed with 1 volume of a 40% sucrose solution in 35% acetic acid. 0,15 or 0,2 ml of which were added to the acrylamide-gel (2 ml in a tube 0,5 times 10 cm) containing 5 M urea and 35% acetic acid. This solution was overlayed with 0,1 ml 75% acetic acid and tubes then filled with 10% acetic acid. The solution in both electrode chambers was 10% acetic acid, too. During the first 5 min it was separated with 2 mA/tube, then 3 or 5 h with 4 mA/tube. The protein bands were stained with amido black 10B. For control steril culture medium was investigated, too. Various preparations of freeze-dried M. hyopneumoniae gave identical protein patterns. Nearly identical were the patterns of protein bands from M. suipneumoniae and M. hyopneumoniae; that means identity of the strains or very close relationship (Figs. 1 and 2). This is in agreement with other authors who investigated both strains with serological methods. M. suipneumoniae and M. hyopneumoniae were found to be different as well from M. hyorhinis and M. sp. E9 from M. granularum (Figs. 1 and 2). Between M. hyorhinis and M. sp. E9 less relationship was noted. All these results were in agreement with investigations performed with the aid of Latex agglutination.