We analyzed amplification and expression of the c-erbB-2 gene in human breast cancers. Southern blot hybridization analysis demonstrated amplification of the c-erbB-2 gene in 10 out of 50 tumor DNAs examined. The degree of amplification was three- to twenty-fold relative to normal placenta. The c-erbB-2 protein could be analyzed in 39 tumor tissues of the 50 samples by immuno-blotting, and elevated expression of the c-erbB-2 protein was found in 15 cases. On the other hand, expression of the c-erbB-2 products was not detected in normal breast tissues either by immuno-blotting or by immuno-histological analysis. These data indicate that transcriptional and/or translational activation of c-erbB-2 might occur in some breast cancers in addition to activation by gene amplification. The elevated expression of the c-erbB-2 protein was most strongly correlated with lymph-node metastasis (P less than 0.001), suggesting that c-erbB-2 expression is involved in promotion of the lymph-node metastasis of human breast cancers. Therefore, immuno-histological diagnosis with anti-c-erbB-2 antibody might be useful as an indicator to predict lymph-node involvement in breast cancer.