Paraffin blocks of all axillary lymph nodes from 97 patients with an initial histologic diagnosis of infiltrating ductal carcinoma and negative axillary nodes were recut and stained with two monoclonal antibodies, AE/AE3 (antikeratin) and DF3 (developed against breast cancer cells and reactive with a glycoprotein tumor-associated antigen). Immunohistochemical staining detected occult micrometastases in 20 patients (20.6%). No patient had more than three lymph nodes involved by tumor. Review of the original hematoxylin and eosin-stained sections revealed that foci of tumor were initially overlooked in nine of these cases (9.3%). In the remaining 11 cases (11.3%) the metastatic foci were encountered in the process of recutting the paraffin blocks for immunohistochemical studies. AE1/AE3 proved to be the more effective of the two antibodies in staining micrometastases. After a mean follow-up period of 5.7 years, no significant decrease in survival or increase in tumor recurrence was detected for patients with occult micrometastases as compared to those patients without micrometastases.