A rabbit-specific polyclonal antibody was obtained raised to a synthetic peptide corresponding to the 1238-1255 C-terminal predicted sequence of the c-erbB-2 protein. This antibody was used in an immunohistochemical procedure to detect the c-erbB-2 protein on a series of 88 paraffin-embedded human breast carcinomas. In 14/88 cases (16%) the c-erbB-2 protein was found to be overexpressed (immunohistochemical score > 1) with a good concordance with the previously determined mRNA level (79/88 cases: 90%). Prognostic significance of c-erbB-2 protein overexpression as detected by immunohistochemistry was tested by the log-rank test. The relative risk of relapse is higher for patients with an immunohistochemical score > 1 (p = 0.00002). In a multivariate analysis of the c-erbB-2 immunohistochemical score was the only powerful parameter (p < 1 x 10(-3). In conclusion, this antibody seems to be a valuable tool in estimating the c-erbB-2 protein regarded in our series as a parameter able to identify a subgroup of operable breast cancer patients with a high risk of relapse.