Cu(II) complexes with oligopeptides containing histidyl residue in the second or third position could scarcely activate hydrogen peroxide (H2O2), but they could activate H2O2 to yield hydroxyl radical ((OH)-O-.) in the presence of biological reductants such as cysteine and ascorbic acid. Further, DNA single strand breakage was also observed during the reactions of Cu(II)-glycylglycylhistidylglycine (GGHG) with H2O2 in the presence of same biological reductants.