The widespread use of antibiotics has been responsible for the development of numerous problems including the emergence of multidrug resistant bacteria, increased number of nosocomial- and community-acquired infections, less than optimal patient outcome, and increased health care costs. Of equal concern is the emergence of resistance in clinical isolates to antibiotics that were once considered "standard" with predictable in vitro susceptibility patterns. Such resistance has been especially notable in organisms that are commonly encountered in a variety of infections including, Streptococcus pneumoniae, Staphylococcus aureus. Enterococci, Klebsiella pneumoniae, and Escherichia coli. It is important for the clinical microbiology laboratory to provide the practicing clinician with accurate and timely antimicrobial susceptibility information which requires the application of standardized and approved in vitro testing methods. The laboratory also serves as a sentinel by maintaining an active monitoring and surveillance program in which current in vitro susceptibility patterns can be compared with local, regional, and national data bases.