Immunohistochemical staining with a novel monoclonal antibody, KiS1, which recognizes a cell cycle-associated antigen, was investigated in 142 cases of stage I and II invasive breast carcinoma. KiS1 staining indices were compared with disease-free interval, overall survival, and post-relapse survival. Using a semi-quantitative method of assessment, we found that tumours with a high level of staining (34/142, 24 per cent) had a significantly worse prognosis than those with a low level of staining (108/142, 76 per cent). Significant correlations were found between KiS1 staining and disease-free interval (P < 0.001), overall survival (P < 0.001), and post-relapse survival (P = 0.008). A more time-consuming, quantitative method of assessment gave similar results. Cox multivariate analysis showed these results to be independent of nodal status, histological type, and grade of tumour (P = 0.01). We conclude that KiS1 is a valuable new antibody which affords useful prognostic information in breast carcinoma. As it can be used in formalin-fixed, paraffin-embedded material, it may be of particular use in the study of small lesions such as those identified in the Breast Cancer Screening Programme.