The expression of retinoblastoma gene product (Rb protein) was studied by immunohistochemical analysis of 205 cases of breast cancer. Rb protein was invariably expressed in non-neoblastic breast epithelium, in dysplastic and hyperblastic lesions adjacent to tumours, and none of the breast tumours was totally negative for Rb protein. According to the scoring system used, the expression of Rb protein was abnormal in 36.6% of cases. Abnormal expression of Rb protein was significantly related to grade (P < 0.00004), type (P = 0.0183), margin formation (P = 0.0116), DNA ploidy (P < 0.0002) and nuclear pleomorphism (P < 0.0001). Abnormal expression of Rb protein was related to high S phase fraction (P = 0.004), high mitotic index (P < 0.001) and high morphometric nuclear factor values (P < 0.01). The expression of Rb protein had no prognostic value in univariate or multivariate analysis. The results show that the tumour suppressor gene Rb participates in the growth regulation of breast cancer cells in vivo, but immunohistochemical assessment of the expression of Rb protein has no prognostic significance in clinical breast cancer over already established prognostic factors.