We have studied the characteristics of fusion of large unilamellar vesicles composed of phosphatidate and phosphatidylinositol alone and in mixtures with other naturally occurring phospholipids. Fusion was induced by the addition of Ca2+ or Mg2+ and was monitored by detecting the mixing of aqueous vesicle contents. Release of vesicle contents was measured by dequenching of carboxyfluorescein fluorescence. Aggregation was monitored by 90 degrees light scattering. The results indicated striking differences with respect to the fusion capacity of the different vesicles. Phosphatidate vesicles fuse in the presence of both Ca2+ and Mg2+ at threshold concentration ranges of 0.03-0.1 mM (Ca2+) and 0.07-0.15 mM (Mg2+) depending on the pH of the medium, 8.5-6.0, respectively. In contrast, phosphatidylinositol vesicles do not fuse with either Ca2+ or Mg2+ even at 50 mM concentrations, in spite of aggregation induced by both cations in the range of 5-10 mM. A large difference in terms of fusion capacity is retained even when these two phospholipids are mixed with phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine in 2 : 2 : 4 : 2 molar ratios. The results are discussed in terms of the molecular mechanism of membrane fusion and the possible role of the metabolic interconversion of phosphatidylinositol to phosphatidate as an on-off control system for membrane fusion phenomena involved in secretion.