In whole rat uterus incubated in the presence of [32P]orthophosphate the oestradiol receptor is [32P]phosphorylated on tyrosine. This finding follows our previous observation that in vitro this receptor can be phosphorylated on tyrosine by a uterus kinase that endows the receptor with oestradiol-binding activity. The calf uterus oestradiol receptor interacts with high affinity with 2G8 and 1G2 antiphosphotyrosine antibodies coupled to Sepharose (Kd values of 0.28 and 1.1 nM, respectively). The interaction with 2G8 antibody has been exploited to purify the oestradiol receptor. This interaction disappears after inactivation of the oestradiol receptor by the nuclear phosphatase that hydrolyses phosphotyrosine of the receptor. This fact substantiates the evidence that the oestradiol receptor in uterus is phosphorylated on tyrosine and that this phosphorylation is required for hormone binding to the receptor. The rat liver glucocorticoid receptor also interacts with high affinity with 2G8 antiphosphotyrosine antibody coupled to Sepharose (Kd value of 0.21 nM). This receptor has been purified by using in sequence heparin-Sepharose and antiphosphotyrosine antibody-Sepharose.