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Since the demonstration by Rona and Takahashi in 1911 (1) that a considerable portion of the calcium present in the serum is not diffusible through a semi-permeable membrane, the various calciumfractions in the blood have been extensively studied. It is now recognized that calcium exists in the serum in three distinct forms. One of these, that calcium bound to proteins, comprises the nonultrafiltrable or non-diffusible portion of the serum calcium. The other two forms, ionic calcium and calcium complexed by such small anions as citrate, phosphate, and bicarbonate, are ultrafiltrable and diffusible.Ionic calcium is generally considered to be the physiologically active component of the total serum calcium (20). Recently, however, it has been suggested (21) that the "biologically active" calcium fraction of serum is different from, and probably larger than, the total ionic calcium as determined by the frog heart technique (22), approaching the value for total ultrafiltrable calcium. In any event, a practical method for the routine measurement of actual ionic calcium has yet to be devised. Consequently, a great deal of effort has been directed toward the development of indirect methods for its determination in man. A variety of techniques has been described, but man serum using a new, simple apparatus2 which eliminates most of the disadvantages of previous methods. Using this apparatus we have investigated the various factors which influence the ultrafiltrability of calcium in human serum and have determined the normal range in healthy human subjects. A subsequent paper will describe our findings in diseased states and under experimental conditions. METHODSUltrafiltration apparatus and procedure. The apparatus used in this work has been described very briefly in a previous publication (23). It uses seamless cellophane tubing to contain the serum with a sintered glass support for the membrane and centrifugal force to supply the filtration pressure. Its principal advantages over other equipment are: 1) The atmosphere and the pH can be accurately controlled within the apparatus throughout the ultrafiltration period; 2) there is no source of metallic contamination; 3) membrane breakage is virtually eliminated; 4) the apparatus is easily constructed and can be used in an ordinary laboratory centrifuge; 5) filtration pressure can be controlled by varying centrifuge speed.The ultrafiltration procedure was carried out in the following manner:A strip of Visking Nojax Casingo (size 24/32) about 9 inches long was soaked in distilled water for 10 minutes. It was wiped dry by drawing through a folded gauze sponge repeatedly until no water was visible. One end was knotted and the tubing was doubled (Figure 1) (Corning No. 39570, 25-mm. diameter with 20-mm. disc), and adding a 6-mm. glass tube at an angle near the fritted disc (Figure 1).

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