We have investigated the possibility that extensins (basic, hydroxyproline-rich glycoproteins of the primary cell wall) contribute to cellular adhesion. Soluble [l4C]extensin, obtained by salt-elution from the walls of living spinach (Spinacia oleracea L.) cells, was found to adhere strongly to all surfaces tested, including glass, polypropylene and polycarbonate. The adhesion was not prevented by salts or dilute acids, by urea (a neutral chaotropic agent), or by ascorbate (a reducing agent), which indicates that it was not due solely to ionic bonding, hydrogen-bonding or oxidative coupling. The adhesion was prevented by guanidinium thiocyanate, an ionic chaotropic agent, suggesting that ionic bonding and hydrogen-bonding co-operated in the adhesion. Sodium dodecyl sulphate and two polyanions (poly-L-glutamate and poly-D-galacturonate) each interfered in the adhesion; this is interpreted as being due to their ability to neutralize the positive charge of extensin. Three polycations (poly-L-lysine, poly-L-arginine and poly-L-histidine) each interfered in the adhesion, probably by competing with extensin for binding sites on a glass surface. The data are compatible with the view that extensins are adhesive polymers that could play a role in the binding of cells to each other and to apparently inert surfaces.