Following ozone oxidation of polyester microfibers of 3.5 mu m average diameter and 0.83 m(2)/g specific area, the fiber surface was subjected to graft polymerization of acrylic acid and subsequently immobilized with serologically active proteins including Staphylococcus aureus protein A, a specific antigen, and a specific antibody, The immobilization reaction was mediated by a water-soluble carbodiimide, which allowed formation of a covalent linkage between the ligand proteins and the grafted poly(acrylic acid) chains. The yields of the immobilized ligand proteins were of the order of 1 mg/g fiber. Their binding affinity and capacity to respective specific proteins were studied in vitro from a buffered solution and serum, it was found that the specific proteins were selectively adsorbed with dissociation constants as low as 1 x 10(-6) M, suggesting the adsorption to take place through highly specific protein-protein interaction. An addition of serum albumin did not significantly affect the specific binding, regardless of the ligand proteins. The binding capacity ranged from 1 x 10(-13) to 1 x 10(-11) mol/cm(2), primarily depending on the surface density of the immobilized ligands and the number of their binding sites per molecule.