The concentration of P29, a phosphoprotein related to estradiol receptor (RE), was measured by an immunoradiometric assay in human breast cancer cytosol and the results compared with the levels of RE and progesterone receptors (RP), as measured by the classical ligand-binding assay. Good linear correlation was found between P29 and RE, but not RP. Tumors which were positive for both RE and RP had a higher P29 mean value than RE-negative, RP-negative tumors. Menopausal status of the patients influenced the result, since premenopausal women had lower mean concentrations or were P29-positive less frequently than postmenopausal women. RE-negative, RP-positive tumors tended to have positive P29 levels. The correlation between RE and P29 corroborates results obtained with other techniques and supports a role for P29 in predicting response to hormonal treatment.