The effect of hyperlipidemic rat serum and its fractions on the synthetic functions of embryonic fibroblasts was studied. Moderately hypercholesterolemic sera (100-140 mg/100 ml) stimulated the synthesis of collagen, but not the synthesis of non-collagenous proteins nor the incorporation of glucosamine or cytidine. The stimulating principle was nondialyzable. It was not associated with the isolated total lipoproteins but was found in the infranatant fraction of sera centrifuged at a density of 1.220.