1. Serum lipids, cholesterol, and lipoproteins were determined on 8 normal 7 atherosclerotic, and 8 lipemic individuals in the fasting state. Serum lipids and cholesterol were determined chemically, the lipoprothins by paper electrophoresis. Serum lipoproteins were also performed after incubating at 6.5° and 37° for 24, 48, and 72 hours, and changes in the physical appearance of the serum, and in the concentration and mobility of the β-lipoproteins were observed. In 12 of the above cases, serum protein patterns were noted with the lipoprotein patterns. 2. In many cases a visible turbidity appeared upon incubating at 37°, never upon incubating at 6.5°. Inmost cases this turbidity appeared after incubating for 48 hours, and tended to occur more frequently in atherosclerotic sera than in normal sera. 3. The concentration of β-lipoproteins decreased upon incubating either at 6.5°orat 37°, the decrease being of the same order of magnitude for normal, atherosclerotic, or lipemic sera. 4. Incubating at 37°increased the mobility of the β-lipoproteins (except when a turbidity appeared), the peak shifting from between the β- and γ-globulins to between the β- and γ-globulins. Incubating at 6.5° had no effect on the mobility of the β-lipoprotein. 5. When a turbidity appeared upon incubating at 37°, the mobility of the β-lipoproteins decreased, the peak shifting toward the origin. As the turbidity became marked, a sixth protein fraction appeared, of slower mobility than the γ-globulin. This protein fraction first appeared to form at the expense of the β- and γ-globulins and then at the expense of the α-globulins, all of which decreased. It is considered that the turbidity is due to the formation of "denatured" proteins and lipoproteins which are characterized by a decrease in mobility. 6. No consistent changes were noted in the concentration, stability and mobility of the α-lipoproteins upon incubating at either 6.5° or 37°.