Comparative Ultrafiltration of Calcium and Strontium in Serum.∗ †

Summary Sera of patients and normal individuals have been ultrafiltered through cellophane to determine relative filterability of Sr and Ca, using Sr85 and Ca45 as tracers. In every case, Sr was found to filter more completely. values for 37.5° averaging close to 60% for Sr. and 40% for Ca. calculated on the basis of total serum filtered. None of several variables tested affected relative filterability of the 2 ions. Neither addition of stable Sr nor slight changes in pH. such as might result from manipulation during the experiment, produced significant changes in filterability. There appeared to be a slight negative temperature effect, values for both isotopes being lower at 37.5° than at 23°. although the effect on Ca45 was usually negligible. Results indicate that Ca is more completely bound to protein than is Sr, more Sr remaining in a free form.