We have developed an immunocytochemical staining procedure (ERICA) using a monoclonal antibody to the estrogen receptor (ER) to determine ER status from samples obtained by fine needle aspiration of primary and recurrent breast cancer tissue (cyto-ERICA). ER status was assessable on 214 of 246 smeared aspirates from breast cancer patients. In 143 (66.8%) assessable smears positive nuclear staining was observed but was completely absent in 71 (33.2%) cases. In 107 cases we were able to compare results with those obtained with the quantifiable dextrancoated charcoal (DCC) radioligand binding technique using surgically excised material. We observed qualitative agreement in 53 of 62 (85.5%) of primary specimens and 16 of 16 (100%) recurrent samples compared to the subsequent DCC result on the same sample. Aspirates obtained from new secondary deposits were also assessed and in 16 of 19 (84.2%) cases results agreed with that established previously by DCC on the primary breast tumor. In a further 6 of 10 (60%) cases the cyto-ERICA result obtained from recurrent samples qualitatively agreed with that determined by DCC on a previous recurrent lesion. A comparison of staining of aspirates was also made against frozen tissue sections stained with the monoclonal antibody (tissue-ERICA). Where comparison was made of primary tumor specimens agreement was observed in 40 of 45 (88.9%) of cases while specimens from secondary lesions agreed qualitatively in 14 of 17 (82.3%) of cases. In a small number of samples where tissue-ERICA was performed on an earlier lesion to that aspirated for cyto-ERICA an agreement of 4 of 5 (80%) was observed. This technique shows good sensitivity in demonstrating ER in aspirate specimens, should therefore permit us to determine ER status before surgery for primary breast cancer, and may also mean that surgery for recurrent disease to determine receptor status is no longer necessary.