This research is designed to study the metabolism of essential fatty acids by human cells. The uptake, incorporation, and subsequent modification of 14C linoleic, linolenic, and arachidonic acids by diploid fibroblasts growing in cell culture is being examined. The cells are challenged with exogenous fatty acid mixtures at different concentrations and degrees of unsaturation in order to investigate cellular homeostatic mechanisms for maintaining phospholipid acyl composition. Fibroblasts relatively deficient in linoleate will be utilized to elucidate requirements for and the roles of essential fatty acids in isolated cells. The cell culture is seen as a model system for determining possible cellular effects of manipulation of the degree of unsaturation of dietary fat, now proposed as a component in the prevention and treatment of atherosclerosis.