The patterns of mouse mammary tumor virus (MMTV) integration in the DNA of spontaneous-mammary tumors, salivary glands and livers of DD/Tbr mice were examined using MMTV env, int -1c and int-2c probes. The MMTV env probe revealed 1 to 7 new proviral insertions in all mammary tumors. MMTV integration into int-1 was observed in 10 of 18 mammary tumors, whereas that into int-2 was seen in only 2 of 18 tumors. Of the 13 salivary glands examined, only 3 showed new MMTV proviral integrations, but rearrangement in int-1 or int-2 loci by MMTV was not observed. Immuno-collidal gold electron microscopy revealed the presence of MMTV particles both in mammary tumors and in salivary glands, but no tumors were found to be developed in salivary glands. Taken together these results suggest that salivary glands support MMTV replication, but the virions thus produced may not lead to salivary gland tumorigenesis. It is suggested that the salivary gland is the source of horizontally transmitted MMTV in DD/Tbr mice.