Administration of an active drug to the intact human body leads to a "drug-body" interaction: the drug exerts its effect on the body, the body disposes of the drug. Administration of an antimicrobial drug to a human body infected with pathogenic bacteria leads to a "drug-bacteria" interaction in addition to "drug-body" and "bacteria-body" interactions. Of these complex interacting mechanisms only the disposition effect of the human body on antibacterial drugs and the effect of antimicrobial agents on bacteria have been quantified, analyzed, and are thus predictable. Postulates have been derived from the findings of these studies and have given the rationale for proper antimicrobial therapy under clinical conditions: antimicrobial actions are functions of the duration and concentration of the active antimicrobial principle at the physiological site of action. Optimal therapeutic efforts consider the need for individual treatment with respect to the choice and type of dosage regimen of the antimicrobial agent administered.