The CaCl2 concentration dependence of the rate of actin filament elongation and of the actin monomer concentration at steady state with actin polymer (the critical actin concentration) has been investigated. A relative rate of actin filament elongation from actin polymer intermolecularly cross-linked with N,N'-p-phenylenebis(maleimide) showed a sigmoidal dependence on the concentration of CaCl2 used to induce actin polymerization. This result is shown to be consistent with a model in which only actin monomer containing five equivalently bound Ca2+ ions (Ka = 2 mM-1) is capable of addition to actin polymer. A relative dissociation rate constant for actin monomer removal from polymer was calculated from the product of the critical actin concentration and the relative elongation rate constant and was found to be virtually independent of CaCl2 concentration. The relationship between Ca2+ binding sites on actin and the CaCl2 concentration dependence of the kinetics of actin filament elongation is discussed.