Technical Abstract: Conjugated linoleic acid (CLA; 9c,11t-18:2), a commercially available diet supplement, has been implicated in a variety of beneficial physiological and biochemical effects in humans. It has been demonstrated in rats that 11t-18:1 is converted (via 9 desaturase) to CLA and that both the 9,11- and 10,12-18:2 CLA isomers are produced in conventional but not germ-free rats fed linoleic acid (9c,12c-18:2), the latter conversion due perhaps to an isomerase enzyme. Samples from human metabolism studies in which deuterium-labelled fats were fed were reanalyzed to determine if detectable amounts of deuterium-labelled CLA and CLA metabolites were formed. We found 11-trans-octadecenoate (fed as the triglyceride) was converted (9 desaturase) to CLA, at a CLA enrichment of ca. 30%. The 11-cis-octadecenoate isomer was also converted to CLA but at <10% the concentration of the 11t-18:1. No evidence for conversion of 10-cis- or 10-trans-octadecenoate to the 10,12-CLA isomer (12 desaturase) was found. No evidence for the conversion of 9-cis,12-cis-octadecadienoate CLA (via isomerase enzyme) was found.