Polyion-sensitive membrane electrodes based on the ion-exchanger dinonylnaphthalene sulfonate (DNNS) have previously been developed and used as an end-point detector for the titrimetric determination of heparin, low molecular weight heparins, and pentosan polysulfate (PPS). Herein, we report a similar detection methodology for double stranded DNA (dsDNA (500 bp) and dsDNA (100 bp)) in buffered solution at mu g/mL levels using protamine as a titrant. Further, it is shown that the titration data can be used to estimate the binding constants between protamine and dsDNA. The equilibrium constant K-eq and the number of protamine binding sites (n) per mole of DNA are determined by recasting the data in the form of a Scatchard plot. (c) 2006 Elsevier B.V. All rights reserved.