IN the course of investigations on the fat of rumen contents1, it was observed that the iodine values of C18 unsaturated fatty acid fractions were substantially higher than could be accounted for by the content of diene and triene acids determined by alkali isomerization2. However, the amounts of material isolated were small, and in order to investigate further the acids responsible for this anomaly, the methyl esters of the C18 unsaturated acids from the caul fat of lambs two to three weeks old, earlier described3, were used.