Estrogen receptor (ER) concentrations have been determined in 191 freshly prepared cytosols from breast cancer biopsies using both the monoclonal enzyme immunoassay (ER-EIA) and the dextran-coated charcoal (ER-DCC) methods in a single laboratory. The concentrations of the ER detected using the two methods are highly and significantly correlated (linear regression curve: ER-EIA = 15.5 + 0.82 ER-DCC; r = 0.97). Nevertheless, it may be most correct to interpret the data by resolving the correlation into two lines, one describing the fit for cytosols with low and intermediate concentrations (the first 75% of the distribution of ER values for all primary breast cancers; less than 217 fmol ER/mg cytosol protein) and one describing the fit for cytosols with the highest ER concentrations (i.e., greater than or equal to 217 fmol ER/mg cytosol protein). Using a cutoff limit of 10 fmol/mg cytosol protein to distinguish between ER positive and ER negative biopsies, discrepancies in the classification of ER status were found in only 6% (12 of 191) of the cases using the two different methods. In all 12 cases, the ER concentrations as determined by both methods were in the lower range of receptor concentrations (0-53 fmol/mg cytosol protein). Of the 12 discrepancies, 10 biopsies were classified as ER negative using the ER-DCC method but ER positive using the ER-EIA method. Additional available data for these 10 patients indicate that the ER-EIA assay yielded the more biologically "correct" result. All 10 of these biopsies were either progesterone receptor positive or had nuclear ER. By identifying the outliers of the linear regression curves (points exceeding the 80% confidence interval) of the logarithmically transformed ER concentrations, 9 of the 12 biopsies were identified. Thus, it is unlikely that the observed discrepancies are due to random events in most cases here. Since most of the few deviations observed appear to represent true differences in the sensitivities of the two methods, the ER-EIA method appears to be superior to the ER-DCC method in our hands. The concentration of ER in 47 cytosols stored at -70 degrees C for 3-6 yr was analyzed using the ER-EIA method, and results were compared to the concentration of ER found using the ER-DCC method on freshly prepared cytosols when the biopsies had been received at the laboratory. The linear regression curve of the correlation between ER concentrations determined using the two methods did not differ significantly from that found for the 191 freshly prepared cytosols.