The c-erbB-2 (HER-2/neu) proto-oncogenes is important in oncogenesis and for determination of prognosis in a number of human malignancies. DNA (Southern) hybridization and immunoblotting (Western) techniques are most commonly utilized for determining amplification status and protein expression of this proto-oncogene, respectively. These extraction techniques are often time-consuming, costly, and subject to variability depending on the histological characteristics of the tumor. Paraffin-immunohistochemistry (P-IHC), on the other hand, is time and cost-effective. In addition, this technique may offer enhanced sensitivity and specificity over extraction techniques due to the in situ nature of analysis. In data presented here, 67 cases of human mammary carcinoma were concomitantly assessed for c-erbB-2 gene copy number and oncoprotein expression by dilutional DNA hybridization and P-IHC, respectively. In 64 (95.5%) of 67 cases, high level expression was associated with gene amplification, whereas no detectable expression was associated with a normal diploid gene copy number. In two of the three discrepant cases, P-IHC predicted amplification not corroborated by Southern analysis. In these cases, tumor mass was limited by the intraductal component of the lesion or by an abundance of stromal elements within the specimen. We conclude that P-IHC offers a favorable alternative to Southern analysis in the assessment of c-erbB-2 gene copy number of this oncoprotein in human mammary carcinoma. Furthermore, immunohistochemistry may prove superior to either extraction technique in specimens with limited tumor mass, such as biopsy materials, stroma-rich tumors, or early stage lesions such as intraductal carcinoma.