Surface structures of the genital mycoplasmas Ureaplasma urealyticum and Mycoplasma hominis that are important in the human immune response and pathogenesis of disease are relatively poorly defined. In this study, an unusual antigen complex of U. urealyticum consisting of multiple bands forming a "ladder" pattern after electrophoretic separation was noted. It is similar to the variable V-1 surface antigen of Mycoplasma pulmonis. Data on U. urealyticum are only preliminary, but the ureaplasma antigen, if it proves to be analogous to V-1, may provide the antigenic determinants for distinguishing among serovars or serogroups and correlating them with pathogenicity. Surface proteins of M. hominis were identified with use of 125I surface labeling, [35S]methionine metabolic labeling, and immunoadsorption of rabbit antiserum. Comparison of M. hominis reference strains PG-21 and 4195 showed little homology between surface proteins, although with metabolic labeling they appeared essentially identical. Immunoblotting with patients' sera, using PG-21 as antigen, showed that most reactions were directed to surface proteins and that a 102K antigen (MH1) was recognized by 94% of the sera. MH1 was one of the few surface proteins of PG-21 that appeared to have counterparts in the other six reference strains, making MH1 a prime candidate for reliable and specific detection of M. hominis infection.