We examined the chromatin binding characteristics of estrogen receptor from MCF-7 cells when bound by [3H] estradiol vs. the high affinity antiestrogen [3H]H1285 [4-(N,N-diethylaminoethoxy) 4'-methoxy-alpha-(p-hydroxyphenyl)alpha-ethylstilbene]. Two sublines of MCF-7 cells were used: E-3, which is sensitive to antiestrogens, and RR, which is antiestrogen resistant and was selected for its ability to grow in the presence of tamoxifen. Chromatin was prepared from both E-3 and RR cells, linked covalently to cellulose and deproteinized sequentially by 0-8 M guanidine hydrochloride (Gdn X HCl). The chromatin acceptor activity unmasked by Gdn X HCl was determined using partially purified (30-fold) activated [3H]estradiol- or [3H] H1285-receptor complexes obtained by KCl step elution from DEAE-cellulose columns. With chromatin from E-3 cells, maximal binding (acceptor activity) for [3H]estradiol-receptor complexes prepared from either type of MCF-7 cells (E-3 or RR) was unmasked by 1 and 6 M Gdn X HCl, whereas [3H]H1285-receptor complexes exhibited maximal binding to 1 and 4 M Gdn X HCl-extracted chromatin subfractions. Chromatin prepared from RR cells was similar to that from E-3 cells in its binding activity for [3H]estradiol-receptor complexes. It differed, however, in that [3H]H1285-receptor complexes showed less chromatin acceptor site binding in general to 1-8 M Gdn X HCl-deproteinized RR chromatin, and the binding peak unmasked by 4 M Gdn X HCl was absent in chromatin from these cells. Receptor binding to chromatin was stable and was competitively inhibited by radioinert estradiol- or H1285-receptor complexes (but not by denatured receptors), demonstrating the saturability and specificity of these acceptor sites. Thus, estrogen receptor binds differently to chromatin depending on whether estradiol or an antiestrogen is bound to it. In addition, the acquisition of antiestrogen resistance by the RR subline of MCF-7 cells appears to result from alterations in the state of its chromatin rather than changes in the receptor itself. Finally, the observation that the chromatin from the resistant cells differs from that of the sensitive cells suggests that antiestrogens may be able to inhibit the growth of MCF-7 and other antiestrogen-sensitive cells not only by antagonizing the stimulatory effect of estrogens, but also by exerting some separate effect of their own.