Antimicrobial susceptibility testing is indicated for any organism that contributes to an infectious process warranting antimicrobial chemotherapy, if its susceptibility cannot be reliably predicted from knowledge of the organism?s identity. Standardized in vitro antimicrobial susceptibility tests are also needed in order to evaluate new antimicrobials against specific groups of organisms in comparison with existing agents. Acquired resistance to one or more classes of antimicrobial agents has now emerged in the major mycoplasmal and ureaplasmal species that infect humans, hence the need to establish accurate and reproducible methods to measure antimicrobial activities in vitro with these organisms. This document provides guidelines for performance, interpretation, and quality control of in vitro broth microdilution and agar dilution susceptibility tests for several antimicrobial agents suitable for use against Mycoplasma pneumoniae (M. pneumoniae), Mycoplasma hominis (M. hominis), and Ureaplasma species (Ureaplasma spp). Information in this document includes designated reference strains and the expected minimal inhibitory concentration ranges for specific drugs that should be obtained when they are tested.