Low-pH-induced DNA structural changes were investigated in the pH range 6.8-2.10 by Raman microspectroscopy. Measurements were carried out on calf thymus DNA in the presence of low concentrations of Mg2+ ions. Vibrational spectra are presented in the wavenumber region 500-1650 cm-1. Large changes in the Raman spectra of calf-thymus DNA were observed on lowering the pH value. These are due to protonation and unstacking of the DNA bases during DNA melting and also to changes in the DNA backbone conformation. The intensities of the Raman bands of guanine (681 cm-1), adenine (728 cm-1), thymine (752 cm-1) and cytosine (785 cm-1), typical of the C2-endo-anti conformation of B-DNA, are discussed. The B-form marker near 835 cm-1 and the base vibrations in the higher wavenumber region (1200-1680 cm-1) are analysed. Effects of low pH value upon the protonation mechanism of opening AT and changing the protonation of GC base pairs in DNA are discussed.