Physical properties of poly(L-ornithine) (PLO), a polycation, poly(L-glutamic acid(4)-co-L-tyrosine) (PLEY), a polyanion, and electrospun fibers made of these polymers have been determined and compared. The polymers adopted random coil-like conformations in aqueous feedstocks at neutral pH and in dehydrated cast films and fibers on glass, and the fibers comprised numerous counterions, according to spectral analysis. Adsorption of model proteins and serum proteins onto hydrated and crosslinked fibers depended on the electrical charge of the proteins and the fibers. The surface charge density of the fibers will be comparable to, but less than, the charge density on the outer leaflet of the plasma membrane of usual eukaryotic cells. The present analysis thus advances understanding of cell behavior on electrospun fiber scaffolds, a topic of considerable current interest.