The binding of calcium in phos-pholipid mixtures is very dependent on the pH, it being a quantitative reflection of the types of dissociable groups in the phospho-lipids present. As the pH increases from 6.5 to 8.5, there is a considerable increase in Ca-binding if triphosphoinositide, phosphatidyl-serine, or phosphatidylethanolamine is present. This effect may be of some consequence in certain physiological processes.