Notes: This paper describes an improved method for on-bead conjugation of antibodies that overcomes the limitations of solution-based protocols (requirement for purified, high-concentration antibodies and the need for multiple buffer changes). The method uses HaloTag technology to immobilize protein A and G onto high-capacity magnetic beads. Antibodies are then captured and labeled on-bead. The authors demonstrate that the method is compatible with amine- and thiol-based chemistries as well as with  mouse and human antibody isotypes, both purified and in cell media. Protein G and Protein A-HaloTag fusion proteins were constructed by cloning the coding sequences for the Fc-binding domains between N-terminal HQ and C-teminal HaloTag sequences. To create Protein A and G beads, the purified Protein G-HaloTag and Protein A-HaloTag constructs were covalently attached to Magne HaloTag Beads--magnetic cellulose beads activated with HaloTag ligands. (4586)