The properties of the antiestrogen-binding protein have been examined in mouse tissues, a species in which nonsteroidal antiestrogens are virtually pure agonists. As in other species studied, this protein was distributed in all tissues - highest levels being in the liver. Subcellular fractionation of mouse liver showed that 82% of the antiestrogen-binding protein was associated with the rough endoplasmic reticulum where it was confined to the membranous component. The antiestrogen-binding protein was also present in smooth endoplasmic reticulum, nuclei and cytosol. Its concentration in intact nuclei was at least 10-times higher than levels previously reported in intact rat liver nuclei. Binding of [3H]tamoxifen to the murine antiestrogen-binding protein was of high affinity (Kd = 1 nM) and was inhibited by unsaturated fatty acids and 7-ketocholesterol. In general, cis-isomers of unsaturated fatty acids were more effective binding inhibitors than trans-isomers. The antiestrogen-binding protein solubilized from rough endoplasmic reticulum membranes by the zwitterionic detergent CHAPS, had a molecular mass of approx. 700 kDa and a sedimentation coefficient of about 19 S. [3H]Tamoxifen binding capacity of the solubilized protein was abolished by trypsin and nonspecific proteinases but not by clostripain or Staphylococcus aureus V8 proteinase, suggesting that lysine residue(s) may be involved in [3H]tamoxifen binding.