The decrease of the critical monomer concentration of ADP-actin by millimolar phosphate concentrations has been analysed in terms of equilibrium constants for binding of phosphate ions to ADP-actin. The decrease has been explained by a 10-fold greater affinity of phosphate ions to polymeric ADP-actin (binding constant 100 M-1) than to monomeric ADP-actin (binding constant 10 M-1). Phosphate has an almost identical effect on the critical monomer concentration of the pointed ends of gelsolin-capped actin filaments in the presence of ATP. The phosphate concentration required for half-maximal decrease of the critical monomer concentration of the pointed ends has been determined to be about 15 mM. By using the fluorescent ATP-analogue, 1,N6-ethenoadenosine 5'-triphosphate, phosphate ions have been found to bind also to monomeric ATP-actin, yet with a slightly higher affinity than to monomeric ADP-actin (binding constant 50 M-1).