Formestane (Lentaron (R)) is a second generation selective aromatase inhibitor with an IC50 of 80 nM. Formestane (Lentaron (R)) is used in the treatment of estrogen-receptor positive breast cancer in post-menopausal women. Formestane (Lentaron (R)) is often used to suppress estrogen production from anabolic steroids or prohormones. Formestane (Lentaron (R)) also acts as a prohormone to 4-hydroxytestosterone, an active steroid which displays weak androgenic activity in addition to acting as a mild aromatase inhibitor. Formestane has poor oral bioavailability and as such is no longer popular as many orally active aromatase inhibitors have been identified. Cells were treated with different aromatase inhibitors anastrozole, formestane, exemestane, and letrozole, or antiestrogens tamoxifen and fulvestrant. UMB-1Ca cells showed significant growth inhibition in response to fulvestrant (100 nM, P < 0.0005) and partial growth inhibition in response to letrozole (100 nM, P < 0.005).