DNA aptamers carrying Pt nanoparticles prepared with cisplatin showed peroxidase enzymatic activity while retaining the specific binding ability of the aptamers. Optimal preparation conditions of DNA-Pt complex prepared with cisplatin were investigated on the synthesis at pH 7-11, a reaction time of 1-18 h and 90 degrees C. The enzymatic reaction of DNA-Pt complex obeyed Michaelis-Menten kinetics. K(M) for the DNA-Pt complex was found to be of the same order as K(M) for hemin and hemin-DNA complex, but one order of magnitude higher than that of horseradish peroxidase. A sandwich type of DNA enzyme-linked aptamer assay (DLAA) using DNA-Pt complex successively detected target protein of thrombin. DLAA using DNA-Pt complex fractioned by ultrafiltration membranes having a molecular weight cut-off of 30 000 and 300 000 showed 1.9-times higher sensitivity than DLAA using DNA-Pt complex without fraction. The DNA-Pt complex having specific size was effective for the sensitive detection of thrombin in DLAA.