Using a serum enzyme immunoassay (EIA) kit from Triton diagnostics we detected c-erbB-2 oncoprotein activity in random sera containing highly elevated tumor markers and also in serial specimens from cancer patients expressing elevated oncoprotein activities. Elevated oncoprotein activity was found not only in sera of breast and ovarian carcinomas but also in sera from colorectal, pancreatic, and prostate carcinomas and even from primary hepatoma. Whenever oncoprotein was overexpressed in an individual patient, there was usually an excellent correlation between the oncoprotein activity and the level of dominant tumor marker in serial serum specimens. Based on the size exclusion S-200 column chromatography, we found only a single molecule containing c-erbB-2 oncoprotein activity in pooled sera from cancer patients whereas two oncoproteins slightly different in size were detected in breast tumor tissue cytosol. Using HPLC on a Superose 12 HR column, the serum portion of the oncoprotein was eluted at a position near IgG, suggesting that the extracellular domain of the oncoprotein exists as a dimer in the serum.