We describe affinity partitioning as a preparative method for membranes and membraneous structures such as organelles, cells, and viruses. Biospecific affinity partitioning is carried out in aqueous polymer two-phase systems, commonly with polyethylene glycol and dextran as phase polymers, in an environment compatible with membrane structures. Ideally, two-phase conditions are chosen to partition the bulk of membrane material into one phase, while the affinity ligand, conjugated to the second phase polymer, will selectively pull the membranes to be isolated into this phase. Suitable ligands include lectins, antibodies, and receptor-specific agents. Because the method has so far been used successfully in rather few instances, all using high ligand receptor densities in target membranes, the discussion focuses on factors to be considered when developing affinity partitioning conditions using new ligands.