The phenol-extracted DNA from phage was subjected to various procedures which disrupt the hydrogen bonds, but do not break any of the ester linkages. This denatured DNA reacts rapidly with formaldehyde and, once reacted, there is no tendency to “renature” or re-establish the former hydrogen bonded structure. These formaldehyde-reacted polynucleotide chains have been examined by density-gradient centrifugation and the band width and band form are compared with the same quantities measured on the undenatured molecules. The results indicate that the molecular weight of the single polynucleotide chains is very high, and in no case lower than one-half that of the undenatured molecules from which they are derived. If this is correct, it means that the polynucleotide chains which make up the large undenatured molecules of 62 × 10 6 mol. wt. consist of only two continuously-linked polynucleotide chains. This conclusion bears on current ideas of genetic recombination.