Ureaplasma urealyticum organisms (ureaplasmas), Mycoplasma hominis, M. fermentans, M. primatum, M. Salivarium and M. pneumoniae have been isolated from the genitourinary tract. The first two of these microorganisms are found most frequently. M. hominis is a cause of some cases of postpartum and postabortal fever, acute pyelonephritis and pelvic inflammatory disease. Ureaplasmas have been associated with chorioamnionitis, habitual spontaneous abortion, low birthweight, the urethral syndrome in women, and nongonococcal urethritis (NGU) in men; but the difficulty of proving an etiological relationship is emphasized. However, in NGU there is accumulating evidence to indicate that ureaplasmas cause some cases. Some patients suffering from NGU, from whom ureaplasmas, mycoplasmas and chlamydiae cannot be isolated, respond to tetracycline therapy. This has suggested that a tetracycline-sensitive microorganism might be responsible. In this context, the isolation of a glucose-metabolizing mycoplasma from the genitourinary tracts of 2 of 13 men with NGU is of interest. This mycoplasma, serologically different from all other tested, has the structural and biological features of a pathogenic organism.