The degree of binding of a drug to plasma proteins has a significant effect on its distribution, elimination, and pharmacological effect since only the unbound fraction is available for distribution into extra-vascular space. The binding of DL76 (1-[3-(4-tert-butyl-phenoxy)propyl]piperidine) to bovine serum albumin (BSA) was studied in viitro by equilibrium dialysis at 37 degrees C and pH 7.4 over the concentration range of 0.32-317.18 microM and at a physiological protein concentration of 602 microM. Drug concentrations were determined by validated LC/MS/MS method. Nonlinear regression analyses of the data pointed to a single class of binding sites (m = 1) with a dissociation constant of DL76 equal 49.20 microM. Scatchard plot concave-down curve might indicate positive cooperativity, which was confirmed by the Hill plot with the slope higher than one.