The cyclic peptide phalloidin, one of the toxic components of Amanita phalloides prevented the drop of viscosity of F-actin solutions after the addition of 0.6 M KI and inhibited the ATP splitting of F-actin during sonic vibration. The data concerning ATP splitting are consistent with the assumption (a) that only 1 out of every 3 actin units of the filaments needs to be combined with phalloidin in order to suppress the contribution of these 3 actins to the ATPase activity of the filament and (b) that all actin units of the filaments can combine with phalloidin with a very high affinity. -halloidin did not only stabilize the actin-actin bonds in the F-actin structure but it also increased the rate of polymerization of G-actin to F-actin. The ability of F-actin to activate myosin ATPase was not affected by phalloidin. The tropomyosin-troponin complex did not prevent the stabilizing effect of phalloidin on the F-actin structure.