Potentiometric titration of DNA in the isoionic solutions has been investigated at different concentrations of the polymer. The intrinsic constants pKo of titrable groups corresponding to the two inflections in the titration curves were calculated by taking into account the electrostatic potential of the macroion. The number of ionogenic groups was evaluated from curves of titration. It has been found that the constants of ionization of both ionogenic groups increased with DNA concentration. The number of ionogenic groups corresponding to the first inflection in titration curves is significantly decreased when the concentration of DNA increases. On the other hand, the number of groups corresponding to the second inflection is slightly increased. Spectrophotometric investigations of isoionic solutions of DNA showed that the molar extinction coefficient increases with the concentration of the polymer. The results are discussed in view of the change of the secondary structure of DNA caused by an increase of its concentration in isoionic solutions.