The interaction of γ;-actinin and actin was investigated under various conditions. It has been shown that γ-actinin affects the G-F transformation of actin, causing an increase in the number of actin monomers required to form a nucleus in the initial step of polymerization. Sonicated fragments of F-actin and heavy meromyosin caused the immediate polymerization of actin under the influence of γ-actinin. Therefore, it can be concluded that γ-actinin inhibits the nucleation step of G-F transformation. Actin filaments which were formed in the presence of γ-actinin (F*-actin) were shown to possess certain characteristic properties when compared with control F-actin. These were as follows: F*-actin solution had a high critical concentration; F*-actin showed a high rate of depolymerization; the flow birefringence of F*-actin decreased with time upon incubation in the absence of free ATP; finally, F*-actin was demonstrated to have ATP-splitting activity. These dynamic features of F*-actin were accounted for in terms of an increase in the rate constant of depolymerization in F*-actin under the influence of γ-actinin.