We evaluated a commercially-available disposable device ("Ultrafree," Worthington Diagnostics) for the anaerobic preparation of protein-free ultrafiltrates from serum for measurement of ultrafiltrable calcium. Sufficient filtrate for the analysis is obtained with 10 min from 0.2 to 0.4 mL of serum at room temperature. We assessed these ultrafilters with regard to permeability of calcium citrate, exclusion of proteins, frequency of leakage, and effect of temperature on results. Within-run and day-to-day coefficients of variation for human serum pools were 1.2 and 1.5%, respectively. Reference intervals (in mmol/L) for total (2.16-2.58), ultrafiltrable (1.44-1.67), dialyzable (1.25-1.41), and ionized (1.04-1.25) calcium have been determined for a healthy population of 69 women and 81 men, ages to 18 to 65 years. The device appears to be the most practicable yet available for use in making this measurement.