Three species of mycoplasmas have been implicated in human disease. Mycoplasma pneumoniae, which causes upper respiratory infections, pneumonia, and other complications, is an uncommon isolate from the ororespiratory tract except during outbreaks of disease. Mycoplasma hominis and Ureaplasma urealyticum are the two principal species of genital mycoplasmas. These are responsible for some cases of urogenital infections, and more recent studies show that they are associated with perinatal disorders. Glycolipid antigens of M. pneumoniae dominated serologic studies of mycoplasmas until recently, when protein antigens were isolated. The latter isolations, the production of monoclonal antibodies to attachment proteins, and the demonstration of antibodies to these proteins will make it possible to identify specific pathogenic components involved in infection due to M. pneumoniae and will permit evaluation of the relative roles of the glycolipid and protein antigens. Serologic studies of the genital mycoplasmas have demonstrated seven serotypes of M. hominis and at least 16 serotypes of U. urealyticum, the latter falling into two large serogroups. Type-specific tissue invasion with genital mycoplasmas has been demonstrated, particularly in relation to perinatal morbidity. Surface antigens of genital mycoplasmas are in the early stages of definition and purification. Present data indicate that there are specific protein antigens, but other possible antigenic forms may exist.