c-erbB-3 protein expression was investigated immunohistochemically in a series of 97 malignant breast tumours using the monoclonal antibody RTJ1. Twenty-eight cases (28.8%) showed c-erbB-3 overexpression, 31 cases (32%) showed normal levels of c-erbB-3 and 38 cases (39.2%) were c-erbB-3 negative. c-erbB-3 overexpression was positively but not significantly related to negative lymph node status and survival over a 10-year follow-up period.