The c-erbB-2 mRNA was detected by the S1 nuclease protection assay and Northern blotting in breast cancer tissues. In contrast to the Northern blot analysis which has been used in all recent publications concerning c-erbB-2 expression on the level of RNA, the S1-nuclease protection assay has distinct advantages with respect to sensitivity, reproducibility, and handling of radioactive probes. We compared the expression of c-erbB-2 in 120 breast carcinomas which were operated in the years 1989-1990 on the level of the mRNA (S1 nuclease protection assay) and the protein (immunohistochemistry), respectively. In general, results obtained with both methods were in good agreement. Only minor differences in classification were observed with 18 samples, all of them belonging to either the moderate or the weak c-erbB-2 expression phenotype. In addition, the level of c-erbB-2-protein was investigated by immunohistochemistry in 271 breast carcinomas which were operated in the years 1984-1987. Comparison of the level of c-erbB-2 expression with the patient history indicates that patients whose tumors had already metastasized to the axillary nodes showed a reduced overall and disease-free survival in the group with pronounced expression of the c-erbB-2 protein. Thus the strong expression of c-erbB-2 oncogene in primary breast cancers appears to be an additional and particular prognostic factor in lymph node positive patients.