Although concern for prevention of atherosclerosis has led to advocacy of increased dietary linoleic acid and decreased consumption of saturated fat, the role of essential fatty acids (EFA) in individual human cells is not fully understood, and there is little information on possible changes in membrane lipids in response to diet modification. It is postulated that polyunsaturated fatty acids (linoleic, linolenic and their long-chain derivatives) are necessary components of membrane phospholipids and that normal human cells have homeostatic mechanisms for maintaining phospholipid acyl composition and distribution despite alterations in availability of exogenous fatty acids. To investigate this, cell cultures of diploid human fibroblasts will be fed low levels of radiolabeled fatty acids and their lipids analyzed by thinlayer and gas chromatography. The cells will be challenged with different mixtures of exogenous oleic and linoleic acids to determine whether they selectively incorporate and/or retain particular fatty acids within total cellular lipid and within phospholipid subclasses. Relative EFA scarcity will be obtained by growing the cells in a low serum, low linoleic acid medium. Phospholipid acyl composition and distribution will be examined, as will possible enhanced incorporation and/or retention of linoleic acid under these conditions. Human fibroblasts will also be exposed to moderate levels of exogenous fatty acids in order to compare the distribution of different acyls between membrane phospholipids and excess fatty acids stored as triglyceride.