A protein in starfish sperm head which bundles actin filaments in vitro: purification and characterization. From an extract of starfish sperm heads, a protein was purified using ammonium sulfate fractionation, Sephacryl S-300, hydroxyapatite and Whatman DE 52 columns. Co-sedimentability on low speed centrifugation of this protein with actin filaments was used as an index in the purification. This protein has a molecular weight of 57,000, as judged by SDS-polyacrylamide gel electrophoresis. It lowers the specific viscosity of an actin filament solution, although this effect is abolished under high ionic conditions such as 300 mM NaCl. Electron microscopic observation shows formation of actin filament bundles with a banding pattern of about 10 nm periodicity. Based on these results, we call this protein starfish sperm fascin. Changes in pH and Mg2+ or ATP concentration have no effect on the action of this sperm fascin. Neither the rate of actin polymerization nor that of depolymerization is affected by this protein. The bundles are depolymerized as well by actin-depolymerizing protein (Mabuchi, I. (1981) J. Biochem. 89, 1341-1344) purified from starfish eggs.