The axonally transported high molecular weight protein fodrin, known to be present in the cortical cytoplasm of neurones and other cells, has been purified to homogeneity and several of its biochemical properties have been characterized. Fodrin is an F-actin-binding and cross-linking protein inducing actin gels. It is composed of two nonidentical polypeptide chains (Mr = 240,000 and 235,000) which form a tetrameric complex of a molecular weight close to 930,000. The similarity of fodrin with tetrameric erythrocyte spectrin is directly shown by rotary shadowed molecules both alone and in interaction with F-actin. The gelation and cross-linking activity of fodrin is influenced both by ionic strength and pH in a manner similar to other cross-linking factors. These results strengthen previous concepts concerning the existence of spectrin-related molecules in nonerythroid cells and point to a possible related function in the submembranous microfilament organization in nonmuscle cells.