We have analyzed antigenic variation of seven M. agalactiae wild strains using different sera from naturally infected sheep. Only 30 day sera recognized all surface proteins and inhibited the growth of mycoplasmas. Furthermore, we have observed that two strongly immunogenic proteins: 55 and 35 kDa were digested using 500 micrograms/ml of trypsin. These two bands are immunoprecipitated together with four other proteins but only the 35 kDa protein is recognized by eluted antibodies.