This paper aims at developing novel bioactive fibrous mats for protein immobilization and for protein separation/purification. For this purpose, an amphiphilic triblock copolymer, biotinylated poly(ethylene glycol)-b-poly(L-lactide)-b-poly(L-lysine) was co-electrospun together with poly(L-lactide-co-glycolide) into ultrafine fibers approximately 2 microm in diameter, and a layer of blocking agent was coated on the fiber surfaces to block off possible non-specific binding of proteins. The biotin species retained their ability to specifically recognize and bind streptavidin, and the immobilized streptavidin could further combine with biotinylated antibodies, antigens and other biological moieties. Horseradish peroxidase-labeled streptavidin and fluorescein isothiocyanate-labeled goat globulin were used to detect the immobilizations of streptavidin and rabbit anti-goat IgG(H+L) via enzyme-linked immunoassay and confocal laser scanning microscope, respectively. The immobilized antigen was eluted from the fiber substrate with a glycine/HCl solution and the eluted antigen retained its bioactivity. Therefore, these biotin-carrying composite fibers have a variety of uses, including selective immobilization of functional proteins, antigen/antibody separation and purification, and vaccine preparation.