Nonsteroidal antiestrogens display antagonistic as well as agonistic properties when compared with estrogens. These observations and studies of their molecular interactions suggest that part of their activity is mediated through estrogen receptor (ER)-mediated pathways. However, other data, concerning mainly effects on growth and cellular proliferation, cannot be explained on this basis. Recent investigations have shown binding of these molecules to a number of other intracellular binding sites beside ER. Using different cell variants of the human breast cancer cell line MCF-7 we could confirm the peculiar role of one of these sites, the specific antiestrogen-binding site (ABS). Published data on ABS are critically reviewed.