The c-erbB-2 gene is amplified in a wide variety of different adenocarcinomas. Generally, gene amplification correlates with increased expression of the c-erbB-2 oncoprotein. Previous assays for the c-erbB-2 oncoprotein have been qualitative or semi-quantitative. In this investigation using human breast cancers, c-erbB-2 oncoprotein levels as measured by enzyme-linked immunosorbent assay (ELISA) correlated significantly with semi-quantitation by immunocytochemistry (r = 0.843, P < 0.0001, n = 97). The cut-off point for the ELISA which gave the strongest agreement with immunocytochemistry was 15 units/micrograms protein. It is concluded that detection of c-erbB-2 oncoprotein by ELISA is quantitative and objective.