Because the anion gap is useful in classifying acid-base disorders, quantitative information on this variable should be readily available. Three populations were studied and means and central 90% ranges were determined for the anion gap, calculated as sodium-(chloride + bicarbonate). In 1047 hospitalized patients the mean was 12 mEq/liter, 86 faster volunteers had a mean of 13 mEq/liter with a 90% range of 9--17 mEq/liter, and 203 plasmaphoresis donors had a mean of 11 mEq/liter with a 90% range of 7--16 mEq/liter. Most of this variability can be accounted for by analytical rather than physiological variation. This quantitation of the anion gap is useful for interchecking electrolyte values as an additional and inexpensive means of laboratory quality control. In 73 of 75 abnormal anion gaps, for which the data were verified to be correct, the clinical information provided a reasonable explanaation for the abnormality.