ESTER-FRACTIONATION analysis of certain animal depot and milk fats has shown the presence of small amounts (1–3 per cent) of unsaturated C20 acids1. These acids have generally been regarded as highly unsaturated2,3 and probably similar to the corresponding liver glyceride fatty acids. Experimental difficulties in purifying the residual ester-fractions (in which the C20 acids accumulate, along with polymerized or partially oxidized acids) have prevented more exact identification.