Sixty cases of primary breast carcinoma have been studied using a monoclonal antibody, Ki67, which recognizes an antigen expressed by cells in G1, S, G2, and M phases of the cell cycle but not Go. A Ki67 score (positive cells/total tumour cells) was determined, and possible relationships between this index of cellular proliferation and a number of clinical and pathological parameters were investigated. There was a strong positive correlation between the Ki67 score and mitotic index (p less than 0.001), a weak negative correlation with age (p less than 0.02), and weak positive correlations with histological tumour grade (p less than 0.03), tumour necrosis (p less than 0.01), and cellular reaction (p less than 0.01). No relationship was noted between the Ki67 score and tumour size, nodal status, tumour oestrogen receptor levels, or menopausal status. The Ki67 score may prove to be an objective indicator of biological behaviour and thus be of clinical significance, particularly since it is not strongly related to other clinical and pathological parameters used in predicting outcome in breast carcinoma.