The concentration of c-erbB-2 oncoprotein (HER-2/neu) was examined in the membrane fraction of 196 breast cancer and 39 benign breast specimens using a recently introduced ELISA-assay. The results were compared with established clinical and laboratory prognostic factors, 19% of all breast cancer specimens showed a c-erbB-2 protein level above 10 HNU/micrograms extract protein and were so classified as positive. Considering only the samples of primary tumors the rate was 21%. Surprisingly the rate was only 13.5% in the tissue samples of recurrent disease. Significant correlations were found between high c-erbB-2 protein levels and the tumor diameter, the axillary lymph node status and the progesterone receptor status. In contrast no correlation was found with estrogen receptor or menopausal status. All specimens of benign tumors showed levels < 10 HNU/micrograms extract protein and were classified as negative. The measurement of the c-erbB-2 protein using this ELISA assay was simple to perform and revealed reliable results. Nevertheless the prognostic power of the c-erbB-2 protein seems to be of minor relevance in comparison with established clinical and laboratory parameters, as the positive rates are too small in regard to the much higher recurrence rates of breast cancer. Probably the serum levels of this oncoprotein will be of greater clinical interest in the course of breast cancer.