G-quadruplex DNAs are cyclic arrays of four guanine bases binding by Hoogsteen hydrogen bonds, found in the telomeric regions of chromosomes and in transcriptional regulatory regions of several important oncogenes. Here, we used high resolution atomic force microscopy (AFM) to observe a specific guanine (G) tetrad mediated complex formation of oligonucleotides containing a G-quadruplex motifs (G-ODN) combined with a palindromic sequence under physiological extracellular conditions. These oligonucleotides have been investigated in correlation to their immunostimulatory effects. We observed structural dependence on ion concentration and G-ODN concentration, where high concentration self-assembled DNA networks were formed. (C) 2003 Elsevier Inc. All rights reserved.