The effect of pH on the Mg2+-induced polymerization of rabbit skeletal muscle G-actin at 20 °C was examined. Polymerization data were obtained at various initial concentrations of Mg2+, Ca2+, and G-actin between pH 6 and 7.5. The data were found to fit a kinetic mechanism for actin polymerization previously proposed at pH 8 in which Mg2+binding at a moderate-affinity site on actin induces an isomerization of the protein enabling more favorable nucleation [Frieden, C. (1982) J. Biol. Chem. 257, 2882–2886]. The data also suggest the formation of actin dimers induced by Mg2+binding is over 2 orders of magnitude more favorable at pH 6 than at pH 8. Little effect on trimer formation is found over this pH range. In addition, the conformation induced by nonspecific binding of metal to low-affinity sites becomes more favorable as the pH is lowered. The critical concentration for filament formation is also decreased at lower pH. The kinetic data do not support fragmentation occurring under any of the conditions examined. Furthermore, as Mg2+exchange for Ca2+at a high-affinity site (Kd< 10-9M) fails to alter significantly the polymerization kinetics, Ca2+release from this site appears unnecessary for either the nucleation or the elongation of actin filaments.