Rational antimicrobial therapy depends on the identity of the causative organisms, the location of the infection, and the condition of the host. Selection of antimicrobial therapy is often started before identification of the causative organism is complete. Certain cultural and staining procedures must be instigated prior to therapy in order to isolate the causative organism. Knowledge of the host's physiologic state is necessary to minimize toxicities and/or failures of therapy. Knowledge of synergistic and antagonistic actions of some antimicrobial agents is necessary for optimal results.