Amplification of the neu (or c-erbB-2 or HER) oncogene is relatively frequent in human breast carcinomas. We have raised a polyclonal rabbit serum in order to detect the neu protein product in tissue sections of tumors. This serum specifically reacted with a 185 kilodaltons neu protein in SKBR-3 cells, a mammary carcinoma cell line with amplified neu. Immunohistochemistry on paraffin-embedded sections of tumors in which the neu gene was amplified showed distinct membrane staining of groups of tumor cells. Sections of tumors with normal copy numbers of neu were negative. Lymph node metastases from tumors positive for neu overexpression also showed the membrane staining pattern, whereas lymph node metastases from tumors negative for neu staining never did. Neu amplification is thus associated with neu protein overproduction in tumors and lymph node metastases, and a routine antibody staining technique can discriminate a high level of neu protein expression from levels commonly present in tumors with normal neu copy numbers.