Ion-exchange calcium electrodes represent the first practical method for the direct measurement of ionized calcium [Ca(++)] in biologic fluids. Using both "static" and "flow-through" electrodes, serum [Ca(++)] was within a rather narrow range: 0.94-1.33 mmoles/liter (mean, 1.14 mmoles/liter). Within a given individual, [Ca(++)] varied only about 6% over a several month period. Consistent pH effects on [Ca(++)] were observed in serum and whole blood, [Ca(++)] varying inversely with pH. Less consistent pH effects were also noted in ultrafiltrates, believed to largely represent precipitation of certain calcium complexes from a supersaturated solution. Heparinized whole blood [Ca(++)] was significantly less than in corresponding serum at normal blood pH, related to the formation of a calcium-heparin complex. [Ca(++)] in ultrafiltrates represented a variable fraction (66.7-90.2%) of total diffusible calcium. There was no apparent correlation between serum ionized and total calcium concentrations. Thus, neither serum total calcium nor total ultrafiltrable calcium provided a reliable index of serum [Ca(++)]. Change in serum total calcium was almost totally accounted for by corresponding change in protein-bound calcium [CaProt]. About 81% of [CaProt] was estimated to be bound to albumin and about 19% to globulins. From observed pH, serum protein, and [CaProt] data, a nomogram was developed for estimating [CaProt] without ultrafiltration. Data presented elsewhere indicate that calcium binding by serum proteins obeys the mass-law equation for a monoligand association. This was indicated in the present studies by a close correspondence of observed serum [Ca(++)] values with those predicted by the McLean-Hastings nomogram. While these electrodes allow study of numerous problems not possible previously, they have not been perfected to the same degree of reliability obtainable with current pH electrodes. The commercial (Orion flow-through) electrode is: (a) expensive. (b) requires periodic replacement of membranes, and (c) has not yet been thermostated. As with blood pH measurements. (d) electrode response is logarithmic, i.e. small potential errors generate rather large [Ca(++)] errors. (e) loss of CO(2) should be prevented, and (f) errors due to other cations must be considered under certain conditions. Despite these limitations, we believe the electrode represents a major advance in calcium metabolism.