Amplification and overexpression of the C-ERBB2 oncogene have been associated with a poor prognosis and a lower response to chemotherapy in human breast cancer. In this study, plasma c-erbB2 concentrations were determined using an enzyme immunoassay in patients with breast cancer. The links between c-erbB2 concentration and tumoral response to chemotherapy were established. The patients with a c-erbB2 concentration higher than the cut-off value (27 U/ml) were considered as c-erbB2+. Ten of the 33 metastatic breast cancers were c-erbB2+. No statistically significant difference in response to chemotherapy was noted between c-erbB2+ and c-erbB2- patients (4/10 objective responses versus 10/23). Variations in c-erbB2 concentrations during treatment were not related to response to treatment.