Protein-containing antigens of Mycoplasma hyorhinis and Mycoplasma pulmonis were partially characterized by radioimmunoprecipitation and SDS-polyacrylamide gel electrophoresis (PAGE). Organisms radiolabeled with 125I by the chloramine T method solubilized in detergent were reacted with antisera directed to each organism. Immune complexes were precipitated with Staphylococcus aureus and the molecular weights of labeled antigens were determined by SDS-PAGE with appropriate standards. Antigens precipitated from labeled M. hyorhinis by antisera to this organism had molecular weights ranging from 20,000 to 74,000. Antisera to M. pulmonis precipitated components from this labeled organism with molecular weights ranging from 28,000 to 150,000. Control sera precipitated only background levels of iodinated materials. Cross-reactive antigens shared by these two species of Mycoplasma were provisionally identified by precipitating labeled antigens of one organism with antisera to the other. Prominent cross-reactive antigens from M. hyorhinis had molecular weights of 74,000, 66,000, 58,000, and 39,000. Other components less clearly resolved had molecular weights between 44,000 and 47,000 and between 30,000 and 35,000. Components precipitated from labeled M. pulmonis by antisera to M. hyorhinis had molecular weights of 74,000, 66,000, and 58,000, with other components between 47,000 and 51,000 which were not clearly resolved.