A novel protein factor which reduced the low-shear viscosity of rabbit skeletal muscle actin was purified from a 0.6 M KCl-extract of an insoluble fraction of sea urchin eggs by ammonium sulfate fractionation, gel filtration column chromatography, DNase I column chromatography, and hydroxylapatite column chromatography. This protein factor was shown to be a one-to-one complex of a 20,000-molecular-weight protein and egg actin. This protein complex accelerated the initial rate of actin polymerization, but reduced the steady-state viscosity of F-actin. It inhibited at substoichiometric amounts the elongation of actin filaments on sonicated F-actin fragments and depolymerization of F-actin induced by dilution. In addition, it increased the critical concentration of actin for polymerization. All these effects of this protein complex on actin could be explained by the "capping the barbed end" of the actin filament by the complex. The 20,000-molecular-weight protein which was separated from actin also possessed the barbed end-capping activities, but differed from the complex in that it did not accelerate the polymerization of actin.