The concurrent use of two or more antimicrobial drugs in the treatment of severe bacterial infections has become a progressively more common practice in recent years. In most instances the decision to utilize two or more drugs is based on a clinical judgment made at the bedside with no more direct laboratory data than the routine bacteriologic identification of the infecting microorganism and the bacterial sensitivity tests. In a few hospitals1-3 a systematic effort has been made in individual cases to select by in vitro testing the most effectively bactericidal combination of antimicrobial drugs in an attempt to provide a more rational choice of drugs.
In general, therefore, it may be said that when the clinician chooses to use two antimicrobial drugs, if the decision is based on laboratory data, he will choose either the two drugs to which the infecting microorganism is most susceptible by separate tests, or he