The problems of antibiotic therapy have been multiplied by the increase in the number of commercially available antibiotics and in the number of bacterial strains resistant to one or more of them. Selection of the most effective antibiotic for treatment of a given infection is frequently difficult. Confronted with an infection of uncertain bacteriologic origin, the clinician has several courses open to him. He may attempt to deduce the causative agent from the clinical symptoms and administer the antibiotic known to be effective against most strains of that organism. In some cases this approach is successful, but often the first drug tried is ineffective because of lack of susceptibility of the infecting agent. Valuable time may thus be lost in blind clinical trial, and avoidable complications may occur. To prevent this, many clinicians routinely determine the infecting organism bacteriologically, so that they can select the appropriate antibiotic with greater assurance.