Open-tubular gas-liquid chromatography (GLC) revealed small quantities (total 0.1 %) of two conjugated octadecadienoic fatty acids in the depot fats and cardiac triglycerides of primates fed a diet incorporating 25% of a lard-corn oil (3:1) mixture. The larger component was identified as cis-9,trans-11-octadecadienoic acid, the lesser as trans-9,trans-11-octadecadienoic acid. Both also appear in human depot fat and plausibly could be derived from animal sources such as pork fat, but several pure vegetable oils were also shown to be potential human dietary sources. Relevant gas-liquid chromatographic retention data for the four most common methyl octadeca-dienoates are summarized, and the thin-layer chromatographic behavior on silicic acid impregnated with silver nitrate, or on plain silicic acid when in the form of mercuric adducts, is described.