Although the exact molecular mechanism is not known, b cell differentiation is a process tightly regulated by cell-surface receptors. In our research we sought to examine the physical structure of the b cell receptor (BCR) within a transgenic model, in order to gain insight into this differentiation process. In this work we use mice and murine hybridoma-cell-lines that express the HUG-heavy chain within their antibodies.The HUG-gene is a chimeric transgene that encodes the constant region of the human g-class heavy chain and a dansyl-specific murine variable region. We are interested in finding out whether HUG can substitute for the μ-heavy chain in the BCR and to examine whether HUG associates with the same proteins as μ in the BCR. To this end, we immunoprecipitated Igb, a known component of the pre-BCR and BCR, from bone marrow and spleen cells obtained from mice of different genetic backgrounds. Immunoprecipitation and Western blot analysis allowed us to examine whether HUG and/or μ associate with Igb.