245 breast cancer tissues were analysed immunohistochemically, using monoclonal antibody Ki-67, which spezifically reacts with a nuclear antigen of proliferating cells and represents the so-called growth fraction. According to the number of positive cells, three Ki-score categories (I-III) were established. The results were compared with prognostic variables (Histological grading, DNA-cytophotometry), staging parameters, immunocytochemical (ER-ICA, mPRI), and biochemical (DCC) hormone receptor assay. We found a good correlation between Ki-67 and grading (Bloom and Richardson): G 1-tumors had small growth fractions, in contrast to G-III carcinoma, which showed a high percentage of Ki-Score III. Receptor negative cases had a large growth fraction and receptorpositive ones showed an uniform distribution. The comparison between Ki-67 and tumor spread or DNA-Cytophotometry showed no correlation. The majority of 31 recurrent tumors not only had a large number of positive cells, but also a definite dependence was found between Ki-Score and length of the disease-free period. Ki-67 seems to be an important immunohistochemical marker of breast cancer, which contributes to the individual assessment of the disease and can possibly give predictions on tumor development and response to radiation and systemic therapy.