This monoclonal mouse antibody reacts with human progesterone receptor (PR), a steroid hormone receptor that plays an important role in breast cancer. Historical studies have shown that estrogen receptor (ER) and PR status is correlated with untreated outcome (i.e. prognostic for well-differentiated invasive breast cancer) and with response to anti-hormonal therapy (1). Generally, the absence of ER and PR may indicate early recurrence and poor survival of breast cancer patients, while the presence of ER and PR in breast tumors may indicate potential benefit from tamoxifen and other anti-hormonal therapies. The antibody may be used in the semi-quantitative detection of human progesterone receptor in tissue sections of human breast cancer by immunohistochemistry.