A cell line containing estrogen receptors (MCF-7) and a cell line lacking estrogen receptors (PC-93) were used for a comparison of biochemical and histochemical procedures to detect estrogen receptors. We evaluated three different fluorescent estrogen derivatives: 17 beta-estradiol-6-carboxymethyloxime-bovine serum albumin-fluorescein isothiocyanate, 17 beta-estradiol-17-hemisuccinate-fluoresceinamine, and coumestrol. The main results were: The relative binding affinities of these ligands for the estrogen receptor were between 0.1 and 2% of the affinity of estradiol. Fluorescent staining of the cells showed no relation to the presence of estrogen receptors. Staining was not suppressed with excess estradiol-17 beta, which is known to prevent binding of low affinity ligands to estrogen receptors. Cells with intact membranes were not stained after treatment with the albumin-linked estrogen derivative; only cells with damaged cell membranes were stained. Treatment of cells with 17 beta-estradiol-17-hemisuccinate-fluoresceinamine resulted in a fluorescent labeling of the cytoplasm in intact and artificially damaged cells. Coumestrol caused only fluorescence of the cytoplasm in intact cells. It is concluded that estrogen receptors cannot be detected with these low affinity ligands. Fluorescence of these cells is probably due to binding of the ligands to low affinity binding sites. The presence of these low affinity binding sites appears not to be related to the presence or absence of estrogen receptors and can therefore not be used to discriminate between estrogen receptor-positive and receptor-negative tumor cells.