The antibiotic sensitivity of strains of Ureaplasma urealyticum is measured usually by a metabolism inhibition technique. Antibiotic-resistant strains have been detected, usually not by testing many randomly collected strains but by testing organisms recovered from patients after treatment. On this selection basis about 10% of strains isolated between 1973 and 1976 from patients with nongonococcal urethritis attending sexually transmitted disease clinics in London were resistant to tetracyclines, as were strains isolated in 1983 to 1984. Such resistant strains are not confined to population groups that receive tetracyclines frequently and have been found in widespread geographic locations. About 40% of tetracycline-resistant strains in the London area also are resistant to erythromycin in contrast to about 10% of tetracycline-sensitive strains. Development of resistance to tetracycline, erythromycin, rosaramicin and spectinomycin by a Ureaplasma strain (serovar 2) isolated from a hypogammaglobulinemic patient in the United Kingdom has been seen following successive treatment with these antibiotics, but multiple antibiotic resistance of this kind must be rare.