Interaction between cetyltrimethylammonium bromide (CTAB) and large T4DNA was studied by potentiometric titration using an ion-selective membrane electrode. The data obtained were assessed in terms of recently reported results regarding the higher order structure of T4DNA with the addition of CTAB : individual DNA chains exhibit discrete phase-transition between random coil and compact globule states. The apparent cooperativity represented as a sigmoidal function is attributed to bimodality in the distribution between the elongated coil and compact globule states of T4DNA chains. In addition, the globule-coil transition of single T4DNA is reversible, demonstrated by single-molecule observation with fluorescence microscopy. At a fixed concentration of CTAB, transition from the globule to the coil state is brought about by the addition of polyacrylic acid, suggesting that this reversible transition may be used for the purification of giant DNA molecules.