lambda-DNA Hind III was denatured by HCl, renatured by NaOH, and completely digested by DNase I sequentially. The produced mixture was separated by gel filtration, and one component was obtained which showed resistance to the digestion of DNase 1. The results of atomic force microscopy (AFM) studies on this component indicated that an unusual form of DNA was presented, with the height 13 times that of double-stranded DNA. The renatured and partially digested lambda-DNA Hind III was also studied by AFM, and the images revealed both the duplex DNA and the unusual DNA. By comparison with known results, it is suggested that the unusual DNA is a multistranded DNA rather than a simple supercoil.