Binding of antimicrobial agents to serum proteins, and to extravascular tissues, affects their distribution, elimination, and pharmacological activity. The extent of binding of drugs to serum proteins is both drug concentration and protein concentration dependent. However, changes in drug concentrations within the therapeutic range have little effect.Drug-protein binding may be reduced by the presence of other drugs, and also by endogenous substances. The latter may be important in cases of impaired renal or hepatic function and in disease states associated with elevated free fatty acids. The greatest reductions in protein binding are observed with drugs that are normally highly bound.Equations are presented to describe the influence of the binding of drugs to serum proteins and tissues on the percentage of drug which is free in the body, the concentration of free and total drug in serum, and the apparent distribution volume of drug in the body. Changes in the percentage bound to serum proteins tend to produce curvilinear changes in the various values with the greatest changes occurring with highly bound drugs (greater than 80% binding). Changes in the percentage bound to tissues tend to produce linear changes in all values except the apparent drug distribution volume. Small changes in tissue binding of highly bound drugs cause marked changes in apparent distribution volumes.Although tissue and extravascular fluid drug levels are primarily influenced by the free drug levels in serum, calculation of actual tissue levels is complicated by variable binding characteristics of specific tissues and tissue fluids, and also by the lipophilic nature of the drug.Renal elimination of antimicrobial agents is markedly influenced by serum protein binding if the major elimination mechanism is glomerular filtration. Serum protein binding has little effect on drug elimination in cases of elimination by renal tubular secretion and hepatic extraction. As in the case of distribution, renal clearance and hepatic extraction may be a function of both protein binding and drug lipophilicity.A drug which is bound to serum proteins has no antibacterial activity in vitro. However, binding of drugs to serum proteins in vivo has major clinical significance only when levels of free drug are reduced to values below the minimum inhibitory concentrations of susceptible micro-organisms. In many cases, high protein binding may be compensated for by greater intrinsic antimicrobial activity of lipophilic drugs.