Actin and actin-binding protein (ABP) have recently been purified from human platelet cytoskeletons (S. Rosenberg, A. Stracher, and R.C. Lucas, 1981, J. Cell Biol. 91:201-211). Here, the effect of ABP on the sedimentation of actin was studied. When ABP was added to preformed F-actin filaments, it bound until a maximum ratio of 1:9 (ABP:actin, mol:mol) was reached. however, when actin was polymerized in the presence of ABP, two and a half times more ABP was able to bind to the actin- that is, every 3.4 actin monomers were now bound by an ABP dimer. ABP was not able to induce the sedimentation of actin under nonpolymerizing conditions but was able to reduce the time and concentration of actin required for sedimentation under slow polymerizing conditions. ABP, therefore, exerts its effect of G-actin by either nucleating polymerization or by cross-linking newly formed oligomers into a more sedimentable form.