Lepidopterans specialised on brassicaceous plants have been shown to possess biochemical adaptations preventing the formation of isothiocyanates. In faeces of generalist lepidopteran larvae glutathione (GSH) and cysteineglycine conjugates of plant-derived isothiocyanates were detected. The presence of GSH conjugates in larval faeces after consumption of crucifers suggests isothiocyanate-detoxification via GST (glutathione-S-transferase) activity in caterpillars. Active native GSTs were purified from midguts of Spodoptera littoralis larvae.It was shown that under conditions widely used for GST characterisation, S. littoralis-GSTs are capable of catalysing the conjugation reaction of GSH with isothiocyanates of different structures.