The influence of diet upon the transport of lipids within the mammalian organism has become an increasingly popular and important subject for study during the last few years because of the growing realization that the average concentration of plasma lipids resulting from a given mode of fat transport over a prolonged period of time bears a relationship to the rate of atherogenesis. Of the many constituents of the diet concerned with fat metabolism and fat transport, dietary fat has been singled out as the chief factor relating diet to sustained hyperlipemia and hypercholesterolemia and, as a presumed consequence, to atherosclerosis. The epidemiological studies of Keys and associates 1 relating dietary fat content to mortality from coronary heart disease in various populations have been criticized recently as showing no association 2 between these variables or showing a less impressive association between coronary artery disease and dietary fat intake than with other dietary constituents, particularly animal protein. 2 " 4 It is my purpose to review the experimental and clinical evidence that protein, particularly animal protein, plays a role in fat transport and may, in certain ranges of intake, play a critical role in determining postabsorptive serum lipid concentrations.LIPID TRANSPORT Before we undertake a discussion of the effect of protein upon fat transport, I should like to review very briefly our current concepts of fat transport as they relate to the function of three organs primarily concerned with this process, i.e., the gut, the liver and the fat depots. The level of serum lipids in the mammal under various conditions of diet and metabolic state reflect principally the interaction of these organs in accomplishing the addition of lipid to or removal of lipid from the plasma compartment. In addition, it is probable that dietary and metabolic factors, including, of course, those operative in diabetes mellitus, exert their influence upon fat transport by modifying the function of one or more of Presented at the Symposium on Fat and Diabetes sponsored