The rate of release of Tl + from unilamellar phospholipid vesicles of various compositions was measured by pulse polarography as a function of prothrombin concentration, in the presence and in the absence of Ca 2 + . The experiments were carried out at different temperatures, in the broad transition temperature region of the lipid mixtures.No significant release was observed from mixtures of 70% DPPC + 30% PC interacting with prothrombin. When it interacted with mixtures of 50% DPPC + 30% PC + 20% PS, the release was facilitated significantly with a first-order release rate constant. It was higher in the presence of Ca 2 + than in its absence, at 24 o C, indicating a more pronounced penetration of the protein molecules in the lipid bilayers with Ca 2 + .