The purification of immunoglobulins was studied by comparing 10 different affinity membranes, prepared by coupling various affinity ligands to different microfiltration membranes. Membranes carrying the synthetic peptide TG19318, histidine, the thiophilic ligand and iminodiacetic acid complexed with Zn(II) showed a weak affinity for human IgG, as expressed by apparent association constants (K-A) in the order of 10(5) M-1. Human IgM and rat IgG bound with high affinity to TG19318 membranes, thus, demonstrating the potential of this sorbent for the purification of immunoglobulins other than human IgG. When carrying Protein-A ligands, membranes based on Nylon 66 coated with low-molar-mass dextran or poly(v inyl alcohol), as well as commercial pre-activated polysulfone (Ultrabind(R)) and regenerated cellulose (Sartobind(R)) membranes, showed high affinity for human IgG (K-A approximate to 10(6) M-1). In contrast, a nylon membrane coated with high-molar-mass dextran yielded only K-A approximate to 10(5) M-1, which was attributed to a low accessibility of the immobilized ligand. Besides the high association constants, Protein-A adsorbers based on polysulfone and regenerated cellulose membranes showed several other advantages, such as enhanced charge-to-charge consistency, simpler preparation procedure, membrane sterilisability, good selectivity for IgG purification from cell culture supernatant and good stability throughout repeated adsorption-elution cycles.