We examined the properties of rabbit skeletal muscle actin labeled at Cys-373 with N-ethylmaleimide or with (iodoacetamido)tetramethylrhodamine, and of N-ethylmaleimide-actin further modified with 7-chloro-4-nitro-2,1,3-benzoxadiazole (which primarily labels Lys-372). All three derivatives polymerize more rapidly than unlabeled actin. As measured by fluorescence photobleaching recovery and low-shear viscometry, all three also show a lower extent of network formation relative to native actin. N-Ethylmaleimide has a much smaller effect on the rate of polymerization and on network formation than do the other two derivatives. We suggest that chemical modification of actin with these compounds may stabilize nuclei, accounting for the acceleration of polymerization. Stabilization of nuclei also reduces the average filament length at equilibrium, thereby reducing the extent of network formation. We note a parallel between these results and the effects that cytochalasin and capping proteins have on the polymerization of actin.