The expression of a novel tumour marker TAG 12 was immunohistochemically determined in paraffin embedded biopsies of 79 female breast carcinomas, with special emphasis on its potential prognostic value. The mean (+/- SD) age of the patients at the time of diagnosis was 58.1 (+/- 13.1) years and all women had been prospectively followed-up for a mean of 12.4 years (range 11.5-13.3 years). All except one tumour were TAG 12 positive. In the normal breast, TAG 12 expression was localized at the apical (secretory) cytoplasm of the epithelial cells. Accordingly, TAG 12 was present in abundance in the intraductal secretions as well. TAG 12 positivity was weakly related to histological grade (p = 0.178) and significantly to tubular grade (p = 0.021) of the tumours. TAG 12 positivity was not significantly related to the following parameters: menopausal status, tumour size, axillary lymph node involvement, histological type of the tumour, DNA ploidy, S-phase fraction or irregularity of tumor cell nuclei. The prediction of lymph node involvement and metastasis was not possible in either pre- or postmenopausal women. Similarly, crude survival, breast cancer survival or disease-free survival were not significantly related to TAG 12 expression. The results indicate that quantitation of immunohistochemically determined TAG 12 expression has no prognostic value in breast cancer. At best, TAG 12 might be of some assistance in discriminating malignant breast lesions from benign conditions.