Integrated sequences of mouse mammary tumor virus (MMTV) have been localized in the genomes of five inbred mouse strains (Balb/c, C3H, DBA/2, A.TH, 129-SV) and one mammary tumor cell line (GR). Two major classes of MMTV sequences have been detected in mouse DNA fractions as obtained by Cs2SO4/BAMD (3,6-bis-(acetatomercurimethyl)dioxane) density gradient centrifugation. The first one corresponds to previously described endogenous sequences (Mtv loci), whereas the second one corresponds to endogenous sequences not previously known, and/or recently acquired; in the case of GR cells exogenous sequences may also be present in this class. The genome distribution is somewhat different for the two classes of sequences, the first one being practically only present in the lightest DNA segments of the mouse genome (GC congruent to 38%); the second one being also represented in heavier segments (GC congruent to 43%). This integration pattern suggests that "ancient" endogenous sequences are practically only localized in genome segments of roughly matching composition, whereas exogenous and recently acquired endogenous MMTV sequences may also be present in heavier fractions.