When a DNA molecule, enzymatically labelled with 32p at one end, is partially digested with a restriction enzyme labelled tdna fragments are obtained which form an overlapping series of molecules, all with a common labelled terminus. ta restriction map can then be constructed from an analysis of the size distribution of these molecules. This technique has been used for the restriction site mapping of cloned histone DNA (h22) where as many as 35 cleavage sites may be accurately determined in a single experiment.