mAb-BGD6 immunoprecipitates an approximately 110-kDa protein under both reduced and nonreduced conditions. RBL-2H3 cells were surface labeled with biotin and immunoprecipitated (IP) with Sepharose beads coupled with NMG, mAb-BGD6, or the anti-FcϵRI mAb-BC4. After extensive washing the immunoprecipitated proteins were detected by blotting with HRP-streptavidin. The 110-kDa protein immunoprecipitated by mAb-BGD6 is distinct from the α subunit of FcϵRI immunoprecipitated by mAb-BC4.