Cathepsin D (CD) has been introduced as a predictor of prognosis in patients with breast cancer due to its mitogenic effect and its role in tumour metastasis. Commonly, the CD content of tumours is examined by means of a biochemical method based upon the use of monoclonal antibodies, and immunohistochemical visualization of CD has not been used extensively. The present study compares the biochemical and immunohistochemical findings in 216 cases of human breast cancer. CD may occur in tumour cells and/or macrophages. Correlation of immunohistochemically determined CD content and biochemical CD content is better in tumour cells than in macrophages. The possible causes of this observation are briefly discussed. Although a statistical correlation between the biochemical and the immunohistochemical CD findings exists, the results in individual cases vary within a wide range. Hence, the results of biochemical and immunohistochemical CD assay in an individual tumour cannot be compared directly.