Bactericidal testing has been used for several decades as a guide for antimicrobial therapy of serious infections. Such testing is most frequently performed when bactericidal antimicrobial agent therapy is considered necessary (such as when treating infectious endocarditis or infection in an immunocompromised host). It has also been used to ensure that the infecting organism is killed by (not tolerant to) usually bactericidal compounds. However, few data are available to support the role of such tests in direct patient care. Several important variables affect the reproducibility of the test results; however, proposed reference methods are now available for performing the MBC test. With minor modifications, these can provide a standardized approach for laboratories that need to perform them. Currently, little evidence is available to support the routine use of such testing for the care of individual patients. However, testing of new (investigational) antimicrobial agents can be beneficial in determining their potential to provide bactericidal antimicrobial activity during clinical use. New methods to assess bactericidal activity are being developed, but as yet none have been rigorously tested in patient care settings; further, for most of these methods, little information is available as to which technical parameters affect their results. In clinical laboratories, all bactericidal tests must be performed with rigorously standardized techniques and adequate controls, bearing in mind the limitations of the currently available test procedures.