Abstract The expression of the c‐ fos protein has been examined in non‐malignant and malignant human breast using immunohistochemistry. Since in some cell types expression is associated with differentiation, while in other cells induction is related to growth, the extent of reactivity in carcinomas has been related to tumour grade, oestrogen receptor, epidermal growth factor receptor, and Ki‐67 staining. There was a greater degree of staining in the carcinomas than in the normal and benign tissue. In the latter, there was no relationship with menopausal status or with extent of proliferative changes. In all instances, staining was nuclear. Poorly differentiated carcinomas had a greater degree of reactivity, but the converse did not apply to the well differentiated tumours. There was no relationship with oestrogen receptor, epidermal growth factor receptor, or Ki‐67 labelling. The greater expression observed in carcinomas correlates with previously reported RNA studies. There is no clear relationship between c‐ fos protein detection and proliferation or differentiation.