Ninety-one tumors (5.6%) containing argyrophilic cells were identified in a series of 1,628 consecutive primary breast carcinomas diagnosed between 1968 and 1972 at the Istituto Nazionale Tumori, Milan, Italy. Histological evaluation of these argyrophilic tumors showed the presence, either throughout the whole tumor mass (pure form) or in some areas (mixed form), of distinctive though not pathognomonic cellular features. Immunocytochemistry revealed the presence of chromogranin A or chromogranin B (secretogranin I) immunoreactivity in 86% of these argyrophilic carcinomas and of neuron-specific enolase (NSE) immunoreactivity in all of them. The three neuroendocrine markers were also immunolocalized at the ultrastructural level in the dense-core granules (granins) and the cytoplasmic matrix (NSE). Immunoblotting studies confirmed the chromogranin A and B and NSE immunoreactivities and documented the presence of secretogranin II. We also studied the relation of the histologic, histochemical, and immunocytochemical features to prognosis. There was no significant correlation between argyrophilia and such clinical parameters as age, menopausal status, tumor size, and overall survival; however, the pure form of argyrophilic tumors had a significant association with less frequent lymph node involvement and a low histologic grade.