The production of monoclonal antibodies against estrogen receptor (ER) and progesterone receptor (PR) has permitted the development of the enzyme immunoassay (EIA) and immunocytochemical assay (ICA) for steroid receptor determination. The results obtained with these two techniques, using the same monoclonal antibodies, were compared in a large series of breast carcinomas (187 for ER and 100 for PR). The correlation between these methods was significant for ER (rs = 0.54) and PR (rs = 0.55) (P less than 0.001) but was lost when the receptor concentrations determined by EIA were less than or equal to 15 and less than or equal to 30 fmol/mg protein for ER and PR, respectively. When these values are considered as cutoffs, the concordance between the two methods was 84.5% for ER and 73% for PR. An analysis of discordant results revealed that low epithelial cellularity generally was present in ICA-positive, EIA-negative specimens, whereas only focal positivity with ICA, or positivity of only normal peripheral mammary ducts and lobules, frequently was found in ICA-negative, EIA-positive tumors. In conclusion, there is good correlation between the results obtained by EIA and ICA methods for detection of ER and PR. The authors suggest that biochemical and histochemical methods for steroid receptors could be considered complementary and used together for the analysis of breast cancer.