The binding of 2-hydroxyestrone (2OH E1), a catecholestrogen which is the main end product of the 2-hydroxylation of estrogen, was investigated in breast cancers. 2OH E1-specific bindings were found in the cytosol (Kd = 0.54 +/- 0.10 nM) and in the endoplasmic reticulum (Kd = 3.36 +/- 1.32 nM). The dissociation rate constants of complexes between [3H]2OH E1 and cytosol or membrane binding sites were 3.30 h-1 and 8.30 h-1 respectively. Qualitative analysis of [3H]2OH E1 cytosolic complexes demonstrated a specific binding component with a mol. wt of 330,000 Daltons. Specificity experiments showed that nonestrogenic hormones were unable to compete with 2OH E1 for its binding sites, whereas triphenylethylene derivatives and catecholamines were potent 2OH E1 competitors. The presence of 2OH E1 specific bindings suggests a potential role of catecholestrogen in breast cancer.