Abstract A GLC method is described for the determination of diethylstilbestrol (DES) in feeds and supplements containing 0.00022-0.0022% DES (2-20 g/ton). A 20 g sample is extracted with an ethanol-chloroform mixture and an aliquot containing 1 or 2 μg DES is taken for assay. The extract is washed with pH 10.5 carbonate buffer and the DES is partitioned into 1N NaOH. After this purification, the DES is reacted in base with dichloroacetyl chloride and the esters formed are simultaneously extracted into chloroform. The DES esters are measured by GLC, using a 5% UC W-98 column and electron affinity detection. Dioctyl tetrachlorophthalate is used as internal standard. The coefficients of variation are on the order of 5% in supplements and 10% in complete feeds.