The occurrence of species-specific and strain-specific antigens in three strains of Mycoplasma arginini (G-230, leonis and 23243) was studied by two-dimensional immunoelectrophoresis. Approximately 20 antigenic components could be detected in each strain. It was possible to analyze 6 to 7 major and distinct components from each strain by two techniques: “enhancement” where antigen to an additional strain is added to the first phase of the electrophoresis which increases the size of common peaks and “suppression” where antiserum to an additional strain is incorporated in the second phase whereby peak size of components to which both sera have antibody are decreased. A total of 10 distinct antigens were recognized. Electrophoretic mobilities relative to bovine albumin ranged from 0.2 to 1.08. Three components were common to all strains; two of these represented major amounts of material. Four components represented strain-specific components. Unique fast components were found both in strains 23243 and G-230. Three antigens were distributed into only two of the three strains. The electrophoretic mobilities of some common antigens were quite different between strains.