The proteolytic fragments of the nuclear estrogen receptor in the MCF-7 cell line were characterized following limited digestion with chymotrypsin and trypsin. Nuclei were isolated from cells previously exposed to 10 nM [3H] estradiol. The proteolytic digestion was performed either on the micrococcal nuclease hydrolysate or on intact nuclei. The molecular weights (Mr) were calculated from the sedimentation coefficients determined on a sucrose gradient and from the Stokes radii estimated by gel filtration. Digestion of the nuclei with micrococcal nuclease solubilized a receptor form of Mr = 151,000. This receptor form was degraded by chymotrypsin to a receptor of Mr = 33,000 and by trypsin to a receptor of Mr = 60,000. Digestion of intact nuclei with chymotrypsin solubilized a receptor form of Mr = 62,000 which dissociated in 0.4 M KCl to a receptor of Mr = 32,000. Digestion of intact nuclei with trypsin followed by micrococcal nuclease solubilized a receptor form of Mr = 75,000 which was further dissociated by 0.4 M KCl to a receptor form of Mr = 60,000. The ability of the receptor forms to bind DNA was tested using DNA-cellulose column chromatography. About 40% of the micrococcal nuclease solubilized receptor form, compared to about 7% of the chymotrypsin degraded receptor and to about 13% of the trypsin degraded receptor forms, all bound to the column and could be eluted by high salt concentrated buffer. We conclude that the nuclear estrogen receptor in the MCF-7 cell line can be partially degraded either in the micrococcal nuclease hydrolysate or in intact nuclei by chymotrypsin or trypsin generating protein moieties, probably receptor fragments of Mr = 33,000 and 60,000 respectively. Both fragments retain their estradiol binding domain and it may be hypothesized that the heavier fragment retains its chromatin binding domain.