The protein binding of a number of basic drugs has been shown to be inhibited when blood is collected in Vacutainer tubes. We found that the plasticizer tris(2-butoxyethyl) phosphate (TBEP), present in plasma collected in Vacutainers, was a potent inhibitor of alprenolol and imipramine protein binding. Its concentration in the plasma could quantitatively explain the displacement phenomenon. Alprenolol binding to a solution of a physiologic concentration (0.67 gm/L, 0.015 mM) of alpha 1-acid glycoprotein (orosomucoid) was decreased from 75% to 16% by addition of 10 microgram/ml (0.026 mM) of TBEP, while imipramine binding was decreased from 69% to 13%. Alprenolol and imipramine binding to albumin and lipoproteins was virtually unchanged by TBEP. Due to its selective effect on binding to alpha 1-acid glycoprotein, TBEP may be a useful tool for studying plasma protein binding of basic drugs.