We have investigated that organization and the distribution of a family of interspersed DNA repeats in the mouse genome. The repeats are at least 5600 base pairs (bp) in size and contain two contiguous BamHI endonuclease fragments, 4000 and 540 bp in size, the larger of which includes a 1350-bp EcoRI fragment studied by previous authors. The repeats are polymorphic in their restriction maps, and represent the major family of interspersed repeats in the mouse genome. The repeats are present almost exclusively in the two light major components of mouse DNA, and the base composition of their large BamHI fragments matches that of those components. The genomic distribution of the repeats is different from that of structural genes, which are present not only in the two light components but also in the two heavy components of mouse DNA. This distribution indicates that the repeats are not involved, at least in any simple way, in the regulation of gene expression.