In solutions that contain several chelating species, or that contain ions such as Zn2+ that compete with Ca2+for binding to BAPTA or EGTA, calculating the free Ca2+ concentration requires simultaneously solving multiple equilibrium expressions. Except for the simplest cases where analytical solutions are feasible, researchers must use an iterative computer program to complete these calculations. MAXCHELATOR, a series of programs for determining free metal concentrations in the presence of chelators,ref is freely accessible on the Web.