A previously published method for determination and confirmation of hexachlorobenzene (HCB) in adipose tissue was also applied to mirex residues. A modified procedure for both residues was collaboratively studied by 12 laboratories. The procedure specifies direct application of an extracted or rendered fat sample to a Florisil cleanup column and one-fraction elution. Mirex and HCB are determined by direct GLC of the concentrated eluate. HCB residues are then confirmed by reaction with isopropanol to form the disubstituted bis-isopropoxytetrachlorobenzene (BITB) derivative. Mirex residues are destroyed by this reaction. All participants were asked to analyze an unknown standard mixture of HCB and mirex and 10 rendered chicken fat samples consisting of one blank and 9 samples fortified with unknown amounts of HCB and mirex. The 9 fortified samples represented 3 samples at each of 3 fortification levels. The HCB fortifications of 20.0, 33.3, and 50.0 ppb yielded average interlaboratory recoveries of 89.6, 87.4, and 92.6%, respectively. The respective coefficients of variation (CV) for HCB results were 9.1, 6.8, and 10.0%. The mirex fortifications of 150, 300, and 500 ppb yielded average interlaboratory recoveries of 89.0, 90.2, and 92.3%, respectively. The respective CV values for mirex results were 7.6,16.5, and 18.1%. The method has been adopted official first action.