The binding interactions of a series of basic ligands with alpha 1-acid glycoprotein (AAG) were examined as a function of pH. The binding to AAG increased with increasing pH, and the binding data were satisfactorily fitted to a model that incorporates the effect of pH and discriminates the association constants of neutral (non-protonated) and protonated forms of ligands. It was shown that ligands in the neutral form have a markedly higher affinity for AAG than the protonated forms, resulting in a concomitant decrease in the pKa of bound ligands. The u.v.-visible difference spectra generated upon binding of a representative ligand to AAG also showed that there was a contribution to the binding arising from the deprotonation of the ligand. It is suggested that all tested ligands bind similarly to AAG and that hydrophobic interactions dominate high-affinity binding to AAG.