A 30% ammonium sulfate precipitation has previously been utilized to partially purify the estrogen receptor(s) of female rat liver cytosol. This procedure has now been used to fractionate the estradiol-binding sites of adult male rat liver cytosol. The 30% ammonium sulfate precipitation partially purifies a group of estradiol-binding sites which have properties quite distinct from the large number of sites present in male liver cytosol. The partially purified male sites seem to have the same properties as the partially purified female estradiol-binding sites. They seem to be proteins that are estrogen specific and have a high estradiol affinity (Kd = 1 X 10(-10) M) and a low estrogen capacity (2.3 fmol/mg liver). The estradiol-binding sites of prepubescent male rat liver cytosol have also been fractionated by 30% ammonium sulfate precipitation. The redissolved ammonium sulfate precipitates from prepubescent male rat liver cytosol contain fewer estradiol-binding sites then those from adult male or female rats. It seems that adult male as well as adult female rat liver contains estrogen receptors.