Avian erythroblastosis virus (AEV) induces both erythroblastosis and fibrosarcomas in susceptible birds. A locus, v-erbB, within the viral genome has been implicated in AEV-mediated oncogenesis. We report here the detection and partial characterization of the protein product of the v-erbB oncogene in AEV-transformed cells. We obtained the antisera necessary for our analysis by expressing a portion of the molecularly cloned v-erbB locus in Escherichia coli and immunizing rabbits with the resulting bacterial erbB polypeptide. Antisera directed against the bacterial polypeptide reacted with v-erbB proteins obtained from virus-infected avian cells. By three criteria--tunicamycin inhibition, lectin binding and metabolic labeling with radioactive sugar precursors--the product of the v-erbB gene appears to be a glycoprotein.