Genomic DNA was obtained from peripheral blood samples of healthy volunteers and interacted with two fluorescent dyes (i.e. Hoechst 33258 and ethidium bromide) in aqueous media. These media containing DNA-dye complexes deposited on the gold coated mica surfaces. Then, STM images were obtained in which the STM was operated in air at atmospheric pressure with a tip-to-substrate bias voltage of 250-1000 mV (sample positive) and the tunneling currents in the range of 10-20 pA by using etched tips of Pt/Ir, in constant current made. Both dyes form molecular clusters on DNA. While, the Hoechst molecules were observed on the DNA chains at regular distances, the ethidium bromide molecular clusters did not. (C) 1998 Elsevier Science B.V. All rights reserved.