Binding equilibria of 12 nonsteroidal, anti-inflammatory substances, salicylic acid, diflunisal, phenylbutazone, azapropazone, fenbufen, biphenylacetic acid, naproxen, flurbiprofen, ibuprofin, diclofenac, indomethacin, and benoxaprofen, to defatted human serum albumin has been investigated at 37 degrees, pH 7.4, in a sodium phosphate buffer, 66 mM, by means of equilibrium dialysis and, in case of salicylic acid, by dialysis rate determinations. Cobinding of each of these drugs with monoacetyl-4,4'-diaminodiphenyl sulfone, warfarin, and diazepam has been studied by measuring dialysis rates of the last-mentioned ligands. Cobinding of each drug with bilirubin was investigated by two techniques, equilibrium dialysis against albumin with and without bilirubin, and by measuring rates of oxidation of free bilirubin with hydrogen peroxide and peroxidase. Results were analyzed in quantitative terms. The use of a site-oriented description versus a stoichiometric analysis is discussed. The stoichiometric description is preferred for the following reasons: (a) Simple relations exist between the percentage of bound drug at low drug concentrations and the first stoichiometric binding constant. (b) The stoichiometric description does not imply that preformed binding sites are present in the albumin molecule. (c) A quantitative, stoichiometric analysis of multiple cobinding of two ligands is possible.