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The studies described in this paper and some which follow have as an objective a better description of serum lipids. The present paper represents the first detailed report of work begun early in 1948, some features of which have been published in abstract (1)(2)(3)(4)(5). Initially our purpose was to extend the observations of Recant, Chargaff and Hanger (6) on the influence of serum proteins upon the cephalin-cholesterol flocculation and the thymol turbidity tests. We included quantitative analyses of the centrifugate for lipids as well as for proteins. It soon became apparent that we were using a procedure which was useful in the study of the structure and function of lipid complexes.As shown in the review by Edsall (7) there has long been evidence that a large proportion of the lipids of the serum occur not as small molecules, but as lipid aggregates, or lipoproteins, of varying complexity. The use of the ultracentrifuge, both quantitative and analytical, has a well established place in the study of serum lipids. Important contributions have been made by McFarlane (8,9), Pedersen (10, 11), the Harvard group (12) ment of rate of flotation when suspended in a standard medium, a technic which we have not used. Principal emphasis in published studies by other workers has been on centrifugation of treated serum. In the present paper we deal exclusively with centrifugation of untreated whole serum. At present it does not seem profitable to attempt a detailed comparison between the results obtained by our procedure on the one hand and those obtained by the system of analysis introduced by Cohn and his associates (21), and the results of the flotation procedures of others. When our studies of diseased states and normal individuals under dietary stress have been presented, such comparisons will be in order. Some of the similarities of our contributions to those of others will be mentioned below in the discussion.The perfect procedure for the study of serum proteins and lipids in the initial step would give complete separation of the different lipid and protein complexes, each entity in pure state with structure unchanged by the procedure effecting separation. We doubt if the procedure described in this communication separates any single lipid or protein complex in pure form; nevertheless, the concentration of complexes at certain levels in the centrifugate is of such a degree as to give new information concerning the structure of the complexes at that level and their concentration in the serum under study. The lipid distribution curves are reproducible within satisfactory limits with normal sera, and are significantly different in certain diseased states, especially in acute virus hepatitis, as we have already reported briefly (3) and will report later in detail. We believe our procedure has three main advantages: The manipulation of serum which effects redistribution of pro-1071 TURNER, SNAVELY, GOLDWATER, RANDOLPH, SPRAGUE, AND UNGLAUB teins and lipids is gentle and involves minimal risk of damage to complex molecul...

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