AlCl3, YCl3, and Pb(NO3)(2) induced a reversible aggregation of vesicles prepared from dipalmitoylphosphatidylcholine (DPPC). The dependence of the aggregation behavior on both the salt concentration and temperature was similar to that previously reported for BeSO4. A nature common to these four salt species is that the metal ions have a strong tendency to be hydrolyzed in aqueous solutions. The mechanism of the DPPC vesicle aggregation induced by these metal ions is considered as follows, When these metal ions are bound to the vesicular surface, they could cause a partial destruction of the hydration shell on the surface of DPPC vesicles because of their high activity for hydrolysis. This would result in the reduction of a repulsive hydration force which is responsible for the stability of phosphatidylcholine vesicles, and accordingly, would lead to an aggregation of vesicles. The dehydration of hydrated water around DPPC headgroups is suggested by the phase-transition behavior of hydrated DPPC bilayer in the presence of these metal ions.