Biliary calcium and gallstone formation. The purpose of this paper is to present a brief overview of the current status of the field of biliary calcium and the role of calcium in the formation and maturation of gallstones. The study of free Ca+(+) ions in bile by electrochemical potentiometric measurements using Ca+(+)-selective ion-exchange electrodes is a relatively new field, but much progress has been made in the past few years. Using this powerful analytical tool, new concepts and findings have arisen in almost every aspect of biliary calcium. Although the current symposium is targeted primarily toward cholesterol gallstones, there are several areas in which understanding of biliary calcium may significantly contribute to a better understanding of the pathogenesis of cholesterol, as well as "pigment" (calcium salt), gallstones. Five broad areas are considered in relation to biliary calcium: (a) physiology (calcium entry into bile), (b) biophysics (the regulation of biliary free [Ca+(+)] as related to Gibbs-Donnan equilibria, (c) physical chemistry (the physicochemical state of calcium in bile, (d) thermodynamics (calcium solubility in bile), and (e) kinetics (pronucleating and antinucleating factors and metastable states). With more specific reference to cholesterol stones, consideration is also made of (a) the calcium salt "seed" hypothesis in cholesterol stone pathogenesis; (b) the interactions of Ca+(+) with phospholipid-cholesterol vesicles, with consideration of possible structural requirements and (c) thermodynamic and kinetic factors as related to peripheral or "eggshell" calcification of existing cholesterol stones. 