We report that the calf uterine estrogen receptor, prepared in a Tris-molybdate buffer, bound by 10 nM [3H]estradiol and eluted by a KCl gradient from DEAE-cellulose columns, yielded only one very sharp receptor peak. Estrogen receptor prepared in phosphate buffer with molybdate and eluted with KCl also yielded only one sharp peak on DEAE-cellulose. However, if DEAE-Sephadex (with phosphate buffer plus molybdate) was used, the [3H]estradiol-receptor complex eluted with two sharp peaks at approximately 0.21 and 0.25 M KCl (Peaks I and II, respectively). But the high-affinity antiestrogen, [3H]H1285, bound to estrogen receptor, eluted only as Peak I and not as Peak II. Both the [3H]estradiol and [3H]H1285 binding peaks were saturable since they could be eliminated with 200-fold excess estradiol. Therefore, ion exchange chromatography using different resins and/or buffers may be useful for determining physicochemical differences in estrogen versus antiestrogen receptor complexes.