The possible interaction of environmental factors with the endogenous mouse mammary tumor virus (MMTV) genome in the development of mammary tumors in the low-tumor-incidence BALB/c mouse strain was examined. Tumors were induced in virgin female animals by treatment with chemical carcinogen 7,12- dimethylbenz[alpha]anthracene or urethan, with or without prolonged hormonal stimulation, or by X-irradiation. Concomitant hormonal stimulation resulted in increased tumor incidences compared with those induced by chemical carcinogen treatment alone. The frequency of tumor induction by irradiation alone or in combination with urethan or prolactin stimulation was very low. MMTV expression in the mammary tumors was assayed by nucleic acid hybridization and by immunohistochemical staining. Depending upon the treatment group, 0 to 89% of the tumors contained detectable levels of MMTV RNA (>/=0.0005% of the total cellular RNA). Tumors which contained detectable viral transcripts exhibited only low levels of MMTV RNA, which did not appear to represent the accumulation of RNA sequences homologous to the entire MMTV genome; synthesis of MMTV structural proteins was detected in only one tumor. Viral RNA-positive tumors were generally associated with a longer latent period. MMTV RNA expression occurred in tumors classified histologically as adenoacanthomas, as well as in mammary adenocarcinomas, although the cell types in the adenoacanthomas expressing viral RNA were not identified. It does not appear that expression of the endogenous MMTV genome is required for maintenance of all mammary tumors in BALB/c mice, although partial genome expression undetectable by the methods employed cannot be ruled out. Linear regression analyses were performed. The mean time to tumor appearance and the percentage of tumors which were MMTV RNA positive were found to vary linearly as a function of the total dose of 7,12-dimethylbenz[alpha]anthracene administered. The percentage of tumors which were MMTV RNA positive was also shown to be linearly related to the mean time to tumor appearance. These relationships provide a basis for predictions in the BALB/c system related to these parameters.