Cofilin, an actin-binding protein, regulates the rate, nature and extent of assembly of the actin cytoskeleton. Native Phast gels show that the addition of cofilin to an actin-DNase I complex (74 kDa) results in the formation of a ternary complex of 94 kDa indicating an equimolar stoichiometry in the ternary complex. Furthermore, native gels show that the addition of cofilin to a solution containing free actin and actin-DNase I and run at pH 8.3 results in cofilin complexing preferentially to the actin-DNase I complex. Conversely, the addition of DNase I to a solution containing an actin-cofilin complex and free actin results in the preferential binding of DNase I to the actin-cofilin complex. These results show that the affinity of cofilin for actin can be increased when actin forms binary complexes. When native gels were run at pH 6.8 the affinity of cofilin for monomeric actin was greater than for the actin-DNase I complex indicating that the cofilin-actin interaction can be regulated by changes in pH. The addition of cofilin to actin resulted in the polymerisation of actin at pH 6.8 whereas at alkaline pH a stable cofilin-actin binary complex could be formed. The biological implications are discussed.