Specific radioimmune precipitation assays were utilized to demonstrate the presence of precipitating antibodies to mouse mammary tumor virus (MMTV) in the high-spontaneous mammary tumor strains of mice: C3H/HeN+, GR/N, BALB/cfC3H, and C57BL/6 X C3H F1 (hereafter called B6C3F1). Antibody titers in C3H/HeN+ mice increased with age, with highest titers observed in tumor-bearing animals. MMTV-precipitating antibodies were not detectable by radioimmune precipitation assay in low-mammary tumor strains (AKR, BALB/c C57BL/6, and C3H/HeN-) but were detectable in MMTV-inoculated BALB/c mice. Appearance of antibodies preceded palpable tumor formation, and antibody titers were directly correlated to virus dose. Natural antibody to MMTV in C3H/HeN+ and B6C3F1 mice coexists with the murine leukemia virus natural antibody as determined by competition radioimmunoassays.