To ensure the accuracy and reproducibility of immunohistochemical assays for determining HER-2/neu status of patients with breast cancer, a reliable standard for monitoring assay sensitivity is necessary. We optimally fixed and paraffin processed human ovarian and breast carcinoma cell lines SKOV-3, MDA-MB-453, BT-20, and MCF-7 in quantities sufficient to meet the needs of a laboratory for the foreseeable future. The material was tested, alongside HercepTest kit cell lines (DAKO, Carpinteria, CA), by 7 breast cancer centers in the United Kingdom and France with different immunohistochemical assays and markers. The cell lines also were analyzed by fluorescence in situ hybridization (FISH) by 2 centers using HER-2/neu kits. FISH produced 100% agreement between the 2 centers: SKOV-3 and MDA-MB-453 showed HER-2/neu amplification and BT-20 and MCF-7 did not. Immunohistochemical analysis and a common evaluation method produced 100% agreement that SKOV-3 and MCF-7 showed 3+ and zero HER-2/neu overexpression, respectively. For MDA-MB-453, there was 71% (5/7) concordance of 2+ immunohistochemical staining and 86% (6/7) concordance of zero or 1 + staining for BT-20. The cell lines provide a valuable standard for gauging HER-2/neu assay sensitivity irrespective of the antibody, antigen retrieval system, detection system, or method of evaluation used.