Phalloidin reduces the critical concentration [G]c = Kc-1 for the depolymerisation of rabbit muscle actin. Adding 1 equivalent of toxin reduces the [G]c by a factor of 30; adding 2 equivalents reduces the [G]c by a factor of 90. The depolymerisation of actin was measured by the exchangeability of 45Ca and [14C]ADP in equilibrium dialysis. Half dissociation of both the metal ion and the nucleotide were found at the same concentration. From this value we calculate the critical concentration for actin [G]c=1.05 x 10(-6)M. The analogous value in presence of 1 equivalent of toxin is [G]'c=3.7 x 10(-8)M. The dissociation from actin for a labelled phallotoxin, [3H]demethylphalloin, was likewise studied by equilibrium dialysis. The apparent KD for this toxin, as well as for the natural toxin phalloidin, is 3.6 x 10(-8)M. The value is identical to that of [G]'c. This indicates that the dissociation of the toxin and the breakdown of the filaments together with a concomitant release of Ca and ADP are interdependent events.