A recently synthesized fluorescein-labeled estrogen (17FE, 1-(N)-fluoresceinyl-estrone-thiosemicarbazone) interacts with estrogen-target cells like the native hormone and visualizes the uptake, transport, and distribution of estrogen in intact target cells. Moreover, estrogen binding sites are traced by 17FE in cryostat sections of estrogen target tissues as well. Cell and tissue 17FE binding sites fulfill the accepted criteria for specific estrogen receptors (finite binding capacity, high affinity, steroid and tissue specificity). This fluorescent probe allows estrogen receptors to be studied in a wide variety of cell and tissue preparations under varying conditions of physiologic and pathophysiologic interest.