A long-range repulsive force near isolated neurofilaments was detected by exclusion of large molecules and by direct force measurements with atomic force microscopy. Adsorption of isolated native neurofilaments to a solid substrate in a high-salt solution (170 mM NaCl), in the presence of coisolating contaminants, shows that the contaminants are excluded from a zone that extends from 50-100 nm from the core of the filament. Force-distance measurements by AFM show the presence of a weak repulsive force that extends >50 nm from the core of the filament; this repulsive force is absent in homopolymers of neurofilament L or trypsinized native filaments that lack the long sidearms present in native filaments. These results suggest that neurofilament sidearms form an entropic brush, thereby providing a mechanism for maintaining interfilament spacing.