Sodium thiocyanate up to 0.5 M is compatible with a stable estradiol-t-receptor complex during sucrose gradient centrifugation; however, the maximum permissible concentration is 0.1 M during Sephadex G-100 and G-200 chromatography. When NaSCN 0.1 M is added to low-salt cytosol (.apprx. 7 mg of protein/ml) age-dependent aggregation of receptor is inhibited; peaks of estrogen-binding activity in sucrose gradients and on Sephadex chromatography are sharp; and instead of the usual larger molecular states (8S) found in low salt, most of the estrogen receptor is under the following form: 4.1S; Stokes radius, 36 .ANG.; MW 61000; f/f0 [frictional ratio] 1.25; homogeneous at electrofocusing, with isoelectric point at 6.0. When cytosol containing NaSCN 0.1 M is diluted down to 2-3 mg of protein/ml or, only for sucrose gradients, NaSCN concentration is increased to 0.4-0.5 M, the 61,000 dalton species decreases, being substituted, without loss of bound estradiol-t, by the following estrogen-binding entity: 2.8S; Stokes radius, 28 .ANG.; MW 3200; f/f0, 1.44. In the presence of NaSCN, KCl up to 0.4 M does not affect in a significant manner the molecular properties of the above forms. When NaSCN is dialyzed out, most of the receptor reverts to a 8-9S state. When cytosol is preincubated with Ca2+ (4 mM) and KCl (0.4 M) before addition of NaSCN, the above picture is modified only in the following aspects: Sephadex chromatography peaks are border and slightly but reproducibly shifted toward higher elution volumes; the electrofocusing pattern consists of a 2-peak heterogeneous band shifted toward higher pH (isoelectric points, 6.4 and 6.6); upon dialysis of NaSCN there is little or no reversion to faster sedimenting states. These modifications appear to depend on limited proteolytic attack of the receptor by Ca2+-activated receptor transforming factor (RTF), not on binding of Ca2+ to receptor. The 4.1S entity is probably a dimer resulting from side-by-side pairing of 2.8S sub-units. Molecular dimension of larger receptor forms purified from cytosol are consistent with the hypothesis that under native conditions in vivo dimers are coupled end-by-end into tetrameric structures with 2 stronger (between subunits) and 2 weaker (between dimers) bonding regions, and tetramers may further self-associate. While NaSCN reversibly releases native dimers and subunits by direct impairment of intersubunit bonds, Ca2+ activated RTF irreversibly and specifically releases slightly modified, about 60,000 MW dimers, by preferential proteolytic attack of the weaker bonding regions and indirect destruction of involved bonds. In vivo this effect of RTF may be instrumental in mobilization and nuclear penetration of receptor-estradiol complex. Heteroassociation of receptor with other proteins of the cytosol is not excluded by the above hypothesis. [Bovine serum albumin, horse muscle myoglobin, horse heart cytochrome c, bovine chymotrypsinogen and human immunoglobulin G were used as reference molecules].