The capacity of Zn2+, Cd2+, or Hg2+ to induce fusion of phospholipid vesicles composed of 50%/50%, 60%/40%, or 80%/20% dipalmitoyl phosphatidylcholine (DPPC)/bovine brain phosphatidylserine (PS) was investigated and compared to that of Ca2+ and Mg2+. In vesicles composed of 50%/50% or 60%/40% DPPC/PS, Zn2+ and Cd2+-induced fusion at concentrations considerably lower than were required for Ca2+-induced fusion. Only limited fusion of 80%/20% DPPC/PS vesicles occurred and Zn2+ was more effective than Ca2+ or Cd2+ in inducing fusion of these vesicles. Mg2+ and Hg2+ did not induce fusion in any of the vesicle systems.