Bovine leukemia virus (BLV) is an oncogenic retrovirus that commonly infects cattle and causes a B cell leukemia/lymphoma in ‰ of 1% of infected cattle. BLV is present in much of marketed beef and dairy products, and breast cancer incidence is greatest in countries with high consumption of bovine foodstuffs. We were therefore interested in determining whether humans were infected with BLV, and whether it might play a role in breast cancer. In previous studies we found that many humans had antibodies to BLV envelope glycoprotein (gp51) and capsid protein (p24), suggesting humans might possibly be infected with BLV. We used immunohistochemistry (IHC) and in situ PCR (IS-PCR) to detect viral protein and proviral DNA, respectively, as signs of infection in surgically excised human breast tissue sections. IHC utilized a monoclonal antibody to the BLV p24 capsid protein. IS-PCR utilized primers from the tax region of the BLV genome to amplify a product with directly incorporated digoxigenin-11 dUTP tags, which were then detected with a peroxidase-conjugated antibody to digoxigenin. The majority of the breast tissues had evidence of BLV proviral genome and four out of 27 were positive for BLV capsid protein. We are working to accumulate data on enough samples to determine whether infection of breast tissue is associated with the pathologic classification of the tissue. This research was supported by funds from the California Breast Cancer Research Program.