Cell surface expression of epidermal growth factor receptor and Her-2 with nuclear expression of Her-4 in primary osteosarcoma. There is controversy over the role of Her-2 in osteosarcoma, with some investigators reporting association between expression and adverse outcome, whereas others point to the lack of gene amplification and membranous expression by immunohistochemistry (IHC) as inconsistent with biological significance. Her-2 normally requires pairing with epidermal growth factor receptor (EGFR), Her-3, or Her-4, but these have been less well studied in osteosarcoma. We evaluated the expression of each of these receptors in osteosarcoma and their potential to contribute to pathogenesis by examining a panel of low-passage primary osteosarcoma cell lines, comparing these with archival tumor specimens. Her-2 immunoreactivity was seen frequently in the diffuse staining pattern described previously. We observed EGFR in all samples by IHC. Her-3 expression was not observed. Her-4 expression was nuclear in distribution in all tumor samples and many cell line samples, consistent with activation and cleavage of the receptor. Quantified expression of Her-2 and EGFR mRNA by quantitative, real-time PCR in cell lines correlated with IHC for Her-2 but not for EGFR. Western blot identified full-length receptors for EGFR and Her-2 in all expected cell lines and showed Her-4 to be predominantly in the p80 form. Flow cytometry identified cell surface Her-2 and EGFR in all lines with receptor expression by IHC. We conclude that the cell surface expression of Her-2 and EGFR and the nuclear localization of the activated p80 fragment of Her-4 suggest that all three may be contributing to osteosarcoma pathogenesis. Therapy directed against this family of receptors may be beneficial for patients with osteosarcoma.