The techniques associated with immunohistochemistry are of major interest in modern tumour diagnosis. Their simple application provides a wider scope for diagnosis by increasing the possibilities of characterising tumour cells. In particular, the combination of various tumour markers has led to a better understanding of tumour histogenesis. If the primary tumour is unknown and only metastases are available for diagnosis, further important information can be gained allowing a correlation with the origin of the metastases. In three own cases an occult carcinoma of the female breast could be diagnosed pre-operatively via immunohistochemistry techniques. Since no marker is available that reacts specifically with cells of metastasizing mammary cancer, lymph node metastases have been examined with several different markers. For that purpose, the epithelial membrane antigen (EMA), Thomsen-Friedenreich-antigen (T-Ag) and alpha-lactalbumin (ALA) have been used to point out several pathways of cell metabolism. It could be demonstrated that the prospective use of the three markers is of the highest value to make diagnosis easier and safer than by interpreting the morphological appearance. The diagnostic value of tumour markers is an important step forward, especially in the interpretation of tumour cell embolism, macrometastases and micrometastases in regional lymph nodes.