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Binding of bumetanide, a loop diuretic, to partially purified albumins from renal failure patients (RF-HA), and healthy subjects (N-HA), human serum albumin (HSA) and defatted-HSA (D-HSA), was studied with equilibrium dialysis at a constant albumin concentration and various ligand concentrations. Binding parameters (n and K) were estimated from Scatchard plots and with a non-linear two-binding site model computer program, assuming two classes of independent sites. The binding capacities (n1K1) decreased in the order N-HA > RF-HA > D-HSA > HSA. Computer estimates of K1 for the partially purified albumin preparations were not markedly different. However, the graphical estimate of K1 for N-HA was greater than that for RF-HA. When the degree of binding (r) was plotted as a function of the logarithm of the free bumetanide concentration, an asymptotic plateau was not observed, indicating that the protein binding sites were not saturated. Consequently, the calculated binding estimates may not adequately describe the binding of bumetanide.