Kinetic study was carried out on the aggregation of dimyristoylphosphatidylserine (DMPS) vesicles induced by the addition of divalent cations ( (\mathrm{Mg}^{2+}, \mathrm{Ca}^{2+}, \mathrm{Sr}^{2+}), and (\mathrm{Ba}^{2+}) ) as well as by (\mathrm{pH}) jump, using a stopped-flow method with turbidity detection. With the increase in cation concentration, the initial rate of aggregation, (V_{0}), increased rapidly above the threshold concentration of about (1 \mathrm{~m} M), and reached the maximum velocity, (V_{0, \text { max }}). The increase in (V_{0}) toward (V_{0, \text { max }}) with the concentration was appreciably dependent on the cation species, whereas the value of (V_{0, \text { max }}) was not affected significantly by the cation species. This behavior of aggregation kinetics of DMPS vesicles is well described in terms of the intervesicular interaction potential composed of van der Waals attraction and electrostatic repulsion, without introducing an extra force such as hydration force which is regarded as responsible for the stability of phosphatidylcholine vesicles. The aggregation of DMPS vesicles was also induced when the negative charge on the DMPS head group was eliminated by protonating the serine carboxyl group by mixing the vesicle preparation with acid solution. This observation is in accordance with the above interpretation that no hydration force acts between DMPS vesicles to such an extent that the aggregation kinetics are affected appreciably. 1994 Academic Press, Inc.