Data from immunocytochemical assessment of estrogen receptor (ER) regulation in MCF-7 cells under estrogenic and anti-estrogenic stimulation were compared with those obtained by enzyme immunoassay (Abbott ER-EIA). Similar trends were observed, although ER level variations were less marked when assessed immunocytochemically. We confirmed reports of ER disappearance in the presence of estrogens (Es; E 2 and DES) and pure anti-estrogens (AEs; RU 58,668 and ICI 164,384) as well as its increase with partial AEs (4-OH-TAM and RU 39,119). E 2 -induced ER down-regulation was partly blocked by actinomycin D (AMD), okadaic acid (OK) and cycloheximide (CHX) when assessed by these 2 methods. Down-regulation by pure AEs was not impeded by CHX, indicating that they operate differently from Es (i.e., transformation of ER to a form sensitive to constitutive degradation activity). In situ pre-labeling of the cells with [ 3 H]TAZ indicated that all investigated ligands eliminate pre-existing ER through binding to newly synthetized receptors, since [ 3 H]TAZ co-valently associates with ER; E 2 and RU 58,668 were more effective than 4-OH-TAM in this regard. CHX blocked ER disappearance even in the presence of pure AEs, which is in contrast to the data established with cells not pre-exposed to [ 3 H]TAZ. Nuclear location of [ 3 H]TAZ-ER complexes may explain this discrepancy, since pure AE-ER complexes were reported to be incapable of nuclear translocation.