DNA-Pt complexes have shown novel enzymatic activity as a peroxidase similar to that of horseradish peroxidase in the colorimetric reaction with its substrate. The enzymatic activity of these complexes increased with increasing reaction time and pH in reaction solutions of DNA and K-2[PtCl4]. This enhanced enzymatic activity was attributed to the increase in Pt conjugated to DNA in the complex. The enzymatic activity per unit mole of the DNA-Pt complex was significantly higher for complexes prepared with high molecular weight DNA because the enzymatic activity of the complex per repeat unit of DNA was almost constant for these complexes prepared under the same reaction conditions. All the DNA-Pt complexes in this study prepared with different DNA sequences (i.e., [A](20), [G](20), [C](20), [T](20), and [AG](10)) exhibited peroxidase enzymatic activity. These complexes showed good thermal stability as compared to native horseradish peroxidase.