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Preincubation for 23 hr of confluent chick embryo heart cell cultures with increasing doses of a phospholipidic complex produces a progressive increase of [14C] leucine uptake and of its incorporation into proteins when compared to control cultures maintained with 0.5% foetal calf serum. Conversely, a longer time of exposure (47 and 71 hr) to the same doses of phospholipids exhibits a negative effect on both parameters. The changes in protein specific activity appear to be due to a real different capability of the cells in utilizing the amino acid taken up rather than to the changes in is endogenous availability.