Using a rat liver cytosol source of enzyme trialkyl phosphorothioates have been shown to be substrates of glutathione S-transferases. Using OSS-trimethyl phosphorodithioate (OSS-Me(O] and OOS-trimethyl phosphorothioate (OOS-Me(O] the methyl transferred to the sulphydryl of glutathione is that attached to phosphorus via an oxygen atom. Fractionation of liver cytosol has shown that although the bulk activity is due to the three isozymes (1-1; 3-4; 1.2), OSS-Me(O) is a general substrate for glutathione S-transferases. The specific activity is low compared with the substrates 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene.