It is a matter of debate whether the amplification of c-erbB-2 oncogene or production of the oncoprotein in breast cancers correlate with the presence of lymph node metastasis and with a poor prognosis. This study was aimed at elucidating the immunohistochemical localization of oncogene products which are related to cell growth, c-erbB-2 product, epidermal growth factor receptor (EGFR), c-myc protein and estrogen receptor (ER), in benign and malignant lesions of the breast. Fresh frozen sections of 25 breast cancers and 11 fibroadenomas from Japanese women were studied by indirect immunoperoxidase method with proper fixation. C-erbB-2 product and EGFR were localized on the cell membrane whereas c-myc protein and ER were observed in the nuclei. Immunohistochemical expression of oncogene products and ER were not only observed in the mammary carcinomas but also in the fibroadenomas. However immunoreactivities of EGFR and ER were more frequently seen in the fibroadenomas (p less than 0.05). In breast cancers, the incidence of immunoreactivity for c-erbB-2 was higher in the cases with lymph node metastasis than cases without nodal metastasis (p less than 0.05) and there was reciprocal correlation between the expressions of EGFR and ER (p less than 0.05). Regarding the size of the primary tumour, there was no statistically significant correlation with the expressions of c-erbB-2, EGFR, c-myc or ER. Histological grade correlated only with the expression of ER (p less than 0.05).