Stopped-flow kinetic measurements were carried out for the aggregation of acidic phospholipid vesicles induced by the addition of BeSO4 solution. The phospholipid species examined were dimyristoylphosphatidic acid (DMPA), dimyristoylphosphatidylglycerol (DMPG), and dimyristoylphosphatidylserine (DMPS). For all of these phospholipid vesicles, BeSO4 induced a rapid aggregation at extremely low concentration compared with other alkaline-earth metal salts. By increasing the solution pH progressively, the aggregation-inducing effect of BeSO4 was promoted increasingly, and the profile of the initial velocity of the aggregation with respect to the salt concentration approached to that observed for the aggregation induced by La3+ ion. This result demonstrates that the high activity of BeSO4 to induce the aggregation of acidic phospholipid vesicles is attributed to the formation of trivalent complex cation, [Be-3 (OH)(3)](3+), due to the hydrolysis of Be2+ in an aqueous solution.