Multiple intratumoral tissue samples from the primary mass of 30 consecutive invasive breast cancer patients were assayed for estrogen receptor (ER) and progesterone receptor (PR) by the dextran-coated charcoal method following frozen section histopathological examination. Steroid receptor status of each sample was classified as positive (R+) or negative (R-), based only upon quantitative guide lines from the ER/PR results. Four out of 32 (12.5%) of the invasive cancers had an intratumoral sample classified as R+ and one sample as R-. R+ invasive ductal carcinomas has a highly significant degree of tubule formation (P = 0.005) when compared with R- invasive ductal cancers. While the quantitative ER content (r = 0.18) and the degree of quantitative variation in ER content (P = 0.04) did not correlate with the tumor cellularity of the individual samples, tumor cellularity (P = 0.005) and ER content (P = 0.005) were lower in the samples from the tumor border than from the central tumor samples. Variations in ER and PR content may be found on a regional basis within a breast tumor mass resulting from heterogeneity of tumor subpopulations and/or differences in tumor cellularity.