In the UK biotypes of black-grass (Alopecurus myosuroides Huds) showing resistance to both chlorotoluron (CTU) and aryloxyphenoxypropionate graminicides are increasingly being observed. Although the precise mechanisms involved in this resistance have yet to be identi®ed, increased herbicide metabolism has been implicated as being involved in at least some cases of resistance. Glutathione S-transferases (GSTs) are a group of enzymes which have been demonstrated to metabolise herbicides in some plants, and the resistant black-grass biotype Peldon contains approximately double the GST activity towards 1-chloro-2,4-dinitrobenzene (CDNB) of susceptible biotypes. To investigate further the possible role of GSTs in herbicide resistance in black-grass, a puri®cation procedure has been developed for these enzymes. A 27.5 kDa polypeptide possessing GST activity was puri®ed from the susceptible biotype Herbiseed. Puri®cation of GSTs from the resistant biotype Peldon also identi®ed this polypeptide along with an additional 30 kDa polypeptide. An in-vitro kinetic study of both crude and puri®ed GST extracts, and western blot analysis using antisera raised against the 27.5 kDa polypeptide, suggest that the 30 kDa polypeptide may possess GST activity, and is not a precursor of the 27.5 kDa polypeptide. These results are discussed and compared to GST pro®les for other weeds and crops demonstrating herbicide resistance or tolerance.