Sera of breast cancer patients from the United States, India, East Africa and China as well as from their families and from healthy women were assayed for antibodies reactive with the murine mammary tumor virus (MuMTV). Detection was by an enzyme-linked immunoassay recently developed in our laboratory. In the women with breast cancer, 62% of the East Africans, 28% of the Indians, 19% of the Americans but only 5% of the Chinese had antibody to MuMTV. Among healthy women, 21% of the Africans, 3% of the Americans and 5% of the Chinese possessed this antibody. Several family members of breast cancer patients, males as well as females, had increased levels of MuMTV antibody. The MuMTV-reactive antibody was removed by absorption with MuMTV, deglycosylated MuMTV and gp52, the major MuMTV envelope protein. It was not absorbed out by murine leukemia viruses, red blood cells from various species, fetal calf serum or carbohydrates. The results suggest that there may be more than one form of breast cancer, definable by reactivity to MuMTV. The murine mammary tumor virus (MuMTV) is the etiological agent of breast cancer in mice. The induction of mammary tumorigenesis, however, is dependent upon the genetic makeup, hormonal status, and diet of the infected mouse. Thus, even in a situation where a known single cause for breast cancer exists, disease manifestation is subordinate to diverse factors in the individual host.