Using molecular techniques we have isolated a virus present in 40% of American women's breast cancers that is 90 to 95% homologous to the Mouse Mammary Tumor Virus (MMTV) which is causal in mice. It is not in the normal tissue of the same breast, thereby excluding genetic inheritance. Worldwide distribution of virus and of breast cancer mimics the distribution of mouse species with much or little MMTV. The HMTV is infectious in vitro for lymphoid and normal breast cells. It conveys many characteristics of neoplasia to infected cells. Epidemiologic studies and immunologic assessments are essential before proof that this is a cause of breast cancer, but Australian scientists and many others around the world also believe this will happen.