There is increasing concern over the adverse effects of antimicrobial drugs on human intestinal flora, including selection of resistant bacteria and disruption of the barrier effect of the normal resident intestinal flora. Currently, there is no documented evidence that antimicrobial drug residues in foods of animal origin cause adverse human health effects (e.g., prolonging antimicrobial therapy, prolonging hospitalization, predisposition to infection, treatment failure) when present at concentrations currently recognized as safe by regulatory agencies. To protect consumers from adverse health effects, federal programs are charged with the regulation of chemicals and drugs and the detection of chemical and drug residues in foods of animal origin. The Food Animal Residue Avoidance Databank (FARAD) system focuses on published pharmacokinetic information such as the tissue elimination half-lives, clearance rates, and volumes of distribution for those drugs, pesticides, and environmental contaminants that have the greatest potential for persisting in tissues of livestock at slaughter.