We propose to study the specificity, nature, distribution and significance of the antigen(s) in human mammary cells, which cross-react with antibody to mouse mammary tumor virus. The antigens will be characterized for stability during histocytological analysis; for specificity with other murine viruses; for intracellular site of the antigen; and for the cell type(s) which express the antigen. The expression of the antigen(s) will be correlated with staging of the breast diseases and the prognosis and history of the tissue donor. We will attempt to amplify the antigen by growing breast tissues in culture with the ultimate objective of biochemical analysis of the antigen.