Effects of feeding an oil seed supplement treated with formalin upon lipid patterns of blood and synthesis of milk fat were evaluated. Percentages and yields of fatty acids of milk fat with chain lengths between 6 and 16 carbons were decreased while percentages and yields of stearate and linoleate were increased when the lipid supplement was fed. Calculations in cows fed control and supplement, 60% and 80%, respectively, of fatty acids of milk were derived from lipids of blood were supported by arterial-venous differences. Comparisons of the fatty acid compositions of triacylglycerol of plasma and milk fat suggested that triacylglycerol may not be the sole source of linoleate transferred from blood to milk fat. A preliminary evaluation of supplement effects upon lipoprotein patterns of serum indicated two peaks in the low density lipoprotein class and that the increase in total cholesterol of blood caused by feeding lipid supplement is due to increases in cholesterol content of the low density and high density lipoprotein classes.