A recently developed Ca ion electrode was evaluated for use in the routine measurement of [human] serum ionic Ca. Used in conjunction with an expanded scale pH meter, the electrode permits the direct and accurate assay of ionic Ca in fresh serum samples with a minimal expenditure of time and materials. Factors found to be critical for reliable operation include electrode mounting configuration, diluent for the calibration standards, sample preparation, control of pH, uniformity of sample volume, and thermo-stasis. Analytic precision, a function of electrode placement, instrumental stability, and read-out parallax, is routinely excellent. Discrimination and recovery studies indicate that the minimal assayable concentration differences are between 0.2 and 0.3 mg per 100 ml. The electrode appears to be applicable to a wide variety of clinical and nonclinical investigations.