A method for quantitative analysis of conjugated diene unsaturation has been developed utilizing tetracyanoethylene-14C (TCNE-14C) in a Diels-Alder condensation. The amount of C14 found in the Diels-Alder adduct has been shown to be a measure of conjugated diene content. The method has proven successful in analysis of a variety of triglycerides, phospholipids, and peroxidized tissue lipids. In the course of this work, a method for removing the fatty acid substituents from phospholipids using lithium aluminum hydride was developed. TCNE-14C analysis for conjugated dienes in rat liver microsomal lipids after dosing with CCl4 or BrCCl3 has provided conclusive evidence that the increase in ultraviolet absorption at 233 nm of these lipids is due to conjugated dienes.