Serial sections on biopsies of 26 women with invasive breast carcinoma were stained for low molecular weight keratins (LKER), high molecular weight keratins (HKER), epithelial membrane antigen (EMA), vimentin (VIM), carcinoembryonic antigen (mCEA and pCEA), and nuclear estrogen (mER) and progesterone receptors (mPGR), using monoclonal and polyclonal sera. Specified areas were identified on serially sectioned slides and staining reactions were compared among areas as well as among patients. The study concludes: (1) LKER staining was positively related to (a) the percentage of tumor cells with densely stained nuclei, (b) a trabecular mode of stromal invasion, and (c) HKER and EMA staining, and inversely related to (d) VIM staining and gross tumor size. (2) HKER was also inversely related to gross tumor size. (3) VIM staining was positively related to pCEA staining. (4) VIM staining was inversely related to staining ER and PGR. (5) LKER, HKER, and EMA staining was stronger in areas of trabecular rather than confluent areas of stromal invasion within the same biopsy. (6) Staining for ER and PGR was not related to mode of stromal invasion, but showed a strong inverse relationship with mitotic index. Positive staining for LKER may be an indicator of better differentiation together with densely staining nuclei and trabecular mode of stromal invasion, whilst VIM and pCEA appear to be related to features indicative of lack of differentiation. Hormone receptor positivity seems to be strongly related to mitotic activity rather than differentiation.