In breast cancer, DNA amplification of the oncogene c-erbB-2, encoding for the p185 protein, is associated with a poor prognosis. A retrospective study on a population of 220 cases of primary breast cancer permitted a quantitative measure of p185 oncoprotein overexpression by an immunoenzymetric assay and the determination of c-erbB-2 amplification by the Southern blot method. A correlation existed between the two measurements (r=0.85) using the double cut-off: DNA 2 copies and p185 400 U/mg protein, and only 2.7% of the cases were discordant. 13.2% of the tumors showed p185 overexpression. The percentage of tumors overexpressing p185 was significantly different between the groups with amplified and non-amplified c-erbB-2. We observed a significant correlation between p185 levels and tumor grade (p=0.03), and an inverse correlation with hormonal receptors (p=0.0001). The p185 assay could be an additional prognostic factor to better define patient subgroups with node negative, grade II, and positive or negative hormonal receptors.