Questionnaire results from 5233 clinical microbiology laboratories participating in the College of American Pathologists (CAP) survey program in the United States were used to establish current standards of practice with respect to in vitro susceptibility testing of Haemophilus influenzae. The results of this CAP survey indicated that the recently developed National Committee for Clinical Laboratory Standards (NCCLS) guidelines for H. influenzae susceptibility tests have been widely adopted, particularly with regard to the medium used to perform susceptibility tests. Haemophilus test medium (HTM) is now the most commonly used medium and there exists a general level of satisfaction (approximately 80%) with medium performance. Specific methodologic recommendations of the NCCLS, however, are often not being followed, for example, length and atmosphere of incubation and means of preparing inocula. beta-Lactamase assays constitute a very commonly employed means of assessing ampicillin activity. Among susceptibility test methods, disk diffusion (82.2%) is much more commonly used compared with broth microdilution (17.8%) procedures. Data are provided regarding the most commonly tested antimicrobials as well as some of the problems encounted when using current NCCLS methods for susceptibility tests with H. influenzae. Finally, the CAP survey questionnaire revealed that many laboratories have applied HTM to susceptibility tests with other fastidious bacteria such as pathogenic Neisseria sp., streptococci, and Moraxella catarrhalis.