The central, intermediate and peripheral regions of 25 breast tumors were analyzed for estrogen receptors (ER) using a combined biochemical (BC)/immunohistochemical (IHC) micromethod. To optimize the regional comparison, the percentage carcinoma per sample (PCS) was evaluated and incorporated into the quantification of the ER. Correction of the measured ER for the PCS eliminated differences in receptor levels between the central and peripheral regions but not the intermediate region. Although the corrected BC-ER level in this region was found to be about 20% higher, the IHC method did not detect such a difference. Determination of the ER status at the actual growth front of a tumor, rather than in the intermediate region where ER levels appear to be highest, may be of greater clinical relevance. Intra-regional ER heterogeneity appears, however, to be particularly pronounced in this area of a tumor.