There are several "non-standardized" test methodologies for performing antimicrobial susceptibility tests on clinical isolates of fastidious aerobic organisms. Critical to the interpretation of a susceptibility test (or any clinical laboratory test) is correlation of the results with the clinical status of the patient. If previous extensive studies have not been performed for a given antimicrobial-organism combination using a specific method, the results must be interpreted with discretion. Even when documented correlative data are available, strict quality control measures must be followed to ensure optimal performance of the test system. Ideally, these include testing of quality control organisms of known susceptibilities similar to the test isolate. Such quality control strains of fastidious organisms are not readily available; however, they may be obtained through local health departments. In our laboratory, in order to inform clinicians of the limitations of the results generated from antimicrobial susceptibility testing of fastidious aerobic bacteria using non-standardized (not NCCLS) procedures, we have adopted a mechanism for reporting results of disc tests as "presumptive" (Figure 1). When reporting dilution test results that are derived using methods other than those described by the NCCLS dilution protocol, we indicate the modifications employed in the particular test (Figure 2). Our goals are to attempt to identify unusual resistance that may occur and to generate results that are as accurate, precise, and meaningful as possible, yet we must be aware of the limitations of the procedures with which we are working (Table V). It is only with these understandings that we can be of service to our clinicians and patients.