Blue Native polyacrylamide gel electrophoresis (BN-PAGE) is a high-resolution method for studying native protein complexes. Here, the migration behaviour of the B cell antigen receptor (BCR) in BN-PAGE with and without prior biotinylation of the cell surface, from which the complexes were purified, are compared. Non-biotinylated complexes appear as a single band, whereas biotinylated complexes display several bands, indicating that biotinylation leads to aggregation of complexes as well as to loss of subunits. Thus, BN-PAGE has limitations in studying multiprotein complexes. These results are used to gain insight into the stoichiometry of the complex.