Filamin, a major high-molecular-weight protein of chicken gizzard smooth muscle, was purified to homogeneity by salt extraction, ammonium sulfate precipitation, agarose gel filtration, and diethylaminoethylcellulose ion-exchange chromatography. Purified filamin is an asymmetric oligomer consisting of two large subunits of identical size (2 X 250 000 daltons) as indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, chemical cross-linking, sedimentation analysis (s10, wo = 10S) and Stokes'radius estimation (a = 120 A), It has no intersubunit disulfide but appears from oxidation studies to have adjacent thiols near the subunit interface. Filamin contains no amino sugars, methylated lysine, methylated histidine, or hydroxyproline, nor does it exhibit myosin-like ATPase activities. Its amino acid composition and physical properties differ from those of gizzard myosin, for which a pruification procedure is described. Filamin and the protein spectrin of erythrocyte membranes have strikingly similar physical properties, but they are chemically distinct.