Hepatic GSH conjugation is the initial step in the mammalian biotransformation of hexachloro-1,3-butadiene (HCBD) and analogous haloalkenes. The present paper reports an in vitro investigation of the glutathione-dependent conversion of HCBD to water-soluble products, i.e. the enzyme-catalyzed conjugation of HCBD with GSH. The method employed avoids artifacts due to the volatility, low solubility and hydrophobic nature of the chloro-carbon substrate. In order to assess the relative importance of membrane-bound and cytosolic glutathione S-transferase in the conjugation process, microsomal and cytosolic fractions from adult rat liver were tested separately for their ability to promote water solubilisation of the substrate. In addition, microsomal purified and liposomally reconstituted glutathione S-transferase, were tested. The reaction exhibited Michaelis-Menten kinetics, and conjugation rates were linear for at least 20 min. The hepatic microsomal fraction metabolized HCBD 116 times faster than the cytosolic fraction when substrate saturated. Both mono- and bis-substituted conjugates were formed by microsomal as well as by the cytosolic fraction. Treatment of animals with inducers and the use of specific inhibitors indicated absence of cytochrome P-450 involvement in the formation of water soluble HCBD metabolites and supported the view that microsomal glutathione S-transferase is more important in catalyzing GSH conjugation of this haloalkene than the cytosolic forms of transferases.