An immunocytochemical technique employing a monoclonal antibody to the estrogen receptor was used to analyze the receptor content in tumor biopsy material from nonpalpable breast carcinomas. Fine needle biopsies were obtained from 12 such tumors using a stereotaxic biopsy technique. The smears were used for cytomorphological diagnosis and receptor analysis. The results show that estrogen receptor status can be determined preoperatively in nonpalpable human breast tumors.