Long interspersed repeated sequences of the mouse genome can be prepared by digesting reassociated DNA with single-strand nuclease. Length resolution reveals many discrete bands that can be assigned to 15 kbp and 6 kbp groups. The reassociated 6 kbp group (which we identify with the MIF-1 family) possesses significant sequence heterogeneity, evidenced by the production of several smaller fragments upon single-strand nuclease digestion of heteroduplexes. The sites of sequence heterogeneity are relatively few and can be mapped using additional restriction endonuclease cuts. We have mapped additional restriction sites into this group, particularly within a cloned HindIII 400 bp fragment, and have also clearly mapped one end of this relatively homogeneous long interspersed repeated sequence.