The current structural model of the B cell antigen receptor (BCR) describes it as a symmetric protein complex in which one membrane-bound immunoglobulin molecule (mIg) is noncovalently bound on each side by an Ig-α/Ig-β heterodimer. Using peptide-tagged Ig-α proteins, blue native polyacrylamide gel electrophoresis (BN–PAGE), and biosynthetical labeling of B cells, we find that the mIg:Ig-α/Ig-β complex has a stoichiometry of 1:1 and not 1:2. An anti-Flag stimulation of B cells coexpressing Flag-tagged and wild-type Ig-α proteins results in the phosphorylation of both Ig-α proteins, suggesting that on the surface of living B cells, several BCR monomers are in contact with each other. A BN–PAGE analysis after limited detergent lysis provides further evidence for an oligomeric BCR structure.