The accurate determination of sex steroid receptors at the time of mastectomy (MX) for breast carcinoma is important for the determination of subsequent therapy of patients who develop metastases in inaccessible sites. The estrogen (E) and progesterone (P) receptor (R) proteins are heat labile, and measured levels may be vulnerable to alterations once the tumor is devascularized. To evaluate potential differences in ER and PR determinations in tumor tissue acquired at biopsy as compared with tumor from the MX specimen, quantitative analyses of ER (21 patients) and PR (17 patients) were performed on dual samples acquired from the initial biopsy (BX) and the subsequent MX specimen. Receptor concentrations were determined both by sucrose density gradient analysis and titration analysis, and results were expressed as fmol/mg cytosol protein. ER values were classified as receptor-rich (greater than 10 fmol/mg), intermediate (3 to 10 fmol/mg), or receptor-poor (less than 3 fmol/mg); PR values greater than 3 fmol/mg were considered positive. ER BX values were found to be rich or intermediate in 18 patients. When compared with BX values, MX ER values were quantitatively unchanged in 11 patients, lower (MX less than BX) in four patients, and higher in three patients (MX greater than BX). In no patient was the BX ER rich or intermediate and the concomitant MX ER poor. In two patients the PR value was "positive" at BX but "negative" at MX. Accordingly, malignant tissue from a pre-MX biopsy specimen is preferred for receptor analysis although it is apparent that tumor tissue from a properly handled MX specimen is satisfactory for the determination of ER status for clinical purposes.