The Southern transfer technique has been used to provide a generally applicable method for ordering DNA restriction fragments. It involves electrophoresis of partially digested DNA, transfer to nitrocellulose filter paper and annealing to a 32P-labelled fragment. Only those partials containing that particular fragment will reanneal to the probe and produce bands on autoradiography. The size of each partial in the labelled set is the sum of the sizes of the fragment used as probe and of one or more adjacent fragments. Thus the size of the adjacent fragments can be determined from the size increments of this set of partials. The method is illustrated by the mapping of certain BamHI sites on coliphage 186 DNA.