The antiestrogen of the triphenylethylene type, taxomifen (Tx), exerts its inhibitory action on cell multiplication of the human breast cancer cell line MCF-7 in the complete absence of estrogens in the culture medium. These cells contain saturable, high affinity binding sites for Tx and specific for antiestrogens (12,000 sites/cell). The concentration of theses sites is significantly decreased in Tx-resistant variants of this cell line (2700 sites/cell). The concentration of estrogen receptors is similar in this population of variants and in the wild population (43,000 and 48,000 sites/cell, respectively). The concept was supported of a role of these antiestrogen binding sites in the molecular mechanism controlling the multiplication of these breast cancer cells.