Atomic force microscopy (A FM) imaging and subsequent image analysis have been used to measure the ion-specific and ionic strength elects on the adsorption of fibronectin to mica surfaces in buffer solution. Increasing the concentration of monovalent Na+ salt solutions is shown to cause a transition from tightly aggregated and "string-of-heads" structures on the mica surface to well dispersed single-molccule adsorption. Studying the effect of two divalent salts. Ni2- and Ca2+, reveals a dramatic enhancement of fibronectin adsorption to mica in hurler solutions containing Ni2+, hut not for Ca2+. The origin of this ion-specific effect is discussed.