Cofilin, a 21,000 molecular weight protein originally purified from porcine brain that is capable of binding to actin filaments in a molar ratio of the protein to actin monomer of 1:1 in the filament (Nishida et al. (1984) Biochemistry 23, 5307-5313), was purified from porcine kidney in the present study. The two cofilins from brain and kidney were indistinguishable from each other with respect to the mobility on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, the one-dimensional peptide map, and the mode of interaction with actin. Treatment of the actin-cofilin complex with a zero-length cross-linker, 1-ethyl-3-[3-dimethylamino)propyl]carbodiimide (EDC), generated a cross-linked product with an apparent molecular weight of 63,000. Analysis of this product by peptide mapping (Sutoh (1982) Biochemistry 21, 3654-3661) showed that cofilin was cross-linked with the N-terminal segment of actin containing residues 1-12.