During a study of the separation of normal human plasma proteins by a lowtemperature ethanol fractionation procedure (6,22,29), certain components have been isolated in pure form and others in varying degrees of purity. We have studied many of these products, and have attempted to characterize certain of them by physical-chemical methods.Studies of the electrophoretic behavior of plasma and plasma fractions have indicated the presence of at least seven protein components: albumin, fibrinogen, and a1-, 02-, PI-, Pz-, and yglobulins (3,15, 31). Studies in the ultracentrifuge have indicated that four main components are normally present: "albumin" (~2 0 .~ = 4.6), "X-protein" (s = 6),5 "globulin" (s = i ) , and "20-component" (s = 20) (16,18,23). Certain components with sedimentation constants between i and 20 have been observed during studies of plasma fractions. These components have been arbitrarily classified into two groups: those with sedimentation constants between 8 and 11, and those with constants between 12 and 18.Studies of the concentrations of each of these ultracentrifuge components in the various fractions obtained during alcohol fractionation of normal human plasma (6, 22) have been carried out, and are recorded graphically in figure 1. For comparison, me have recorded the distribution of the electrophoretic components in a similar manner in figure 2. The height of each bar represents the weight of that fraction obtained during fractionation. For fraction V several bars are placed side by side and the heights should be added together, this fraction representing 48 per cent of the plasma protein (by weight), or 31.5 g. per liter. A comparison of figures 1 and 2 indicates that there is no simple corre-1 Presented a t the Twentieth Kational Colloid Symposium, which was held a t Madison, Wisconsin, May 28-29, 1946. West Virginia.6 "X-protein", unlike the other components of plasma, was found by Pedersen (23) t o have a specific volume very near unity, and accordingly to have a n uncorrected sedimentation-constant very sensitive to the density of the solvent. I n 0.5 M sodium chloride solution, a sedimentation constant of about 2.9 is observed. Because of this "density effect" of X-protein, i t can easily be differentiated from other components.
