Estrogen receptor concentration was measured in fine needle aspirates from 98 human mammary carcinomas. We performed a biochemical radiolabeled assay by the Charcoal-Dextran procedure on cytosols obtained after homogenization in a saline hypertonic buffer (0,4 M KCl) and centrifugation at 105,000 g. DNA content of the pellets was measured by fluorimetry. A sample of about 10(6) cells, containing at least 10 micrograms of DNA, was needed to obtain a reliable result. In this report, 74 samples were over 10 micrograms of DNA each. The analysis of these samples showed that, in 39 cases (52,8%), the estrogen receptor level was over the limit of 500 fmol/mg of DNA. In 5 cases (6,7%), the range was between 500 to 600 fmol/mg DNA and, in 30 cases (40,5%), it was under 500 fmol/mg DNA and was considered as negative. For 15 patients, estrogen receptors were determined on the same tumor, as removed by fine needle aspiration and by a surgical procedure (6 cases) or drill-biopsy (9 cases). There was a good correlation between the receptor content in needle aspirates and in tissue specimens for 13 cases out of 15.