The activities of aromatase and estrone sulfatase which are important enzymes involved in the local production of estrogen in breast cancer tissue were measured to examine their availability in endocrine therapy and their clinical significance. The materials obtained were breast cancer tissue, noncancerous mammary gland and breast fat tissue from twenty eight patients with breast cancer, and mammary gland tissue from eight patients with benign breast disease. After centrifugation of homogenized tissue at 1000 X g, the supernatant of breast cancer tissue or mammary gland and the subnatant of breast fat tissue were used as enzyme sources. Aromatase activity was measured by 3H2O release assay using (1 beta-3H) androstenedione as the substrate, while estrone sulfatase activity was estimated from the conversion rate of (6,7-3H)estrone-3-sulfate to estrone. Aromatase activities were 25.1 +/- 12.4 (mean +/- S.D.) fmol/mg protein/h in twenty seven breast cancer tissue specimens, 11.0 +/- 6.1 fmol/mg protein/h in sixteen noncancerous mammary gland tissue specimens, 9.3 +/- 10.0 fmol/mg protein/h in twenty seven breast fat tissue specimens, and 7.7 +/- 5.5 fmol/mg protein/h in eight mammary gland tissue specimens from patients with benign breast disease. The aromatase activity in breast cancer tissue was significantly higher than that in noncancerous mammary gland, breast fat tissue and benign breast lesions (p less than 0.001). Estrone sulfatase activity was 4.0 +/- 3.5 nmol/mg protein/h in nineteen breast cancer tissue specimens, but was almost undetectable in eleven noncancerous mammary tissue specimens and eight benign breast lesions. These results suggest the relatively high local production of estrogen, mediated by aromatase or estrone sulfatase in breast cancer tissue.(ABSTRACT TRUNCATED AT 250 WORDS)