Appropriate use of antibiotics is enhanced by the effective application of data on antimicrobial susceptibility. A number of methods are currently available for routine determination of susceptibilities, including agar dilution, broth dilution, agar diffusion, and several semiautomated methods. If susceptibility testing is properly done, it can serve as the basis for the generation of statistical data of use to the clinician. Periodically updated tables detailing the antimicrobial susceptibilities of commonly isolated organisms enable the physician to make appropriate choices of antibiotics for prophylaxis and for the initial treatment of serious infections, before the susceptibilities of the infecting organisms are known. With computer assistance, it is possible to use susceptibility data for quality control and for more sophisticated epidemiologic purposes. For the past several years, we have studied a number of potential uses of computer-generated data at the Massachusetts General Hospital. Several of these applications, especially as they might relate to the prophylactic use of antibiotics, are discussed in this paper.