Estimation of PR and ER by immunocytochemistry in breast cancer. Comparison with radioligand binding methods. Immunocytochemical assays for progesterone receptor (PR) using monoclonal antirabbit PR antibodies (PR-ICA) and for estrogen receptor (ER) (ER-ICA) were compared with radioligand binding (dextran-coated charcoal [DCC]) methods for receptor determination in patients with breast cancer. Immunocytochemical staining for PR was exclusively nuclear in localization. In this regard, PR staining is similar to previous findings for ER; PR-ICA showed a sensitivity of 89% and specificity of 100%. ER-ICA was also 89% sensitive and similarly specific. There was good correlation between the degree and intensity of staining and quantitative binding of radioligand. Receptor-positive tumors, however, show considerable variation of immunocytochemical staining, suggesting heterogeneity of cellular PR content. The availability of an immunocytochemical assay for PR increases the discriminatory potential for these methods of receptor determination. 