A fluorescent dye monochlorobimane (MCB) that binds glutathione (GSH) was used as a tool for measuring the concentration of GSH in skin and mechanically-transformed schistosomula. The specificity of MCB binding to GSH was confirmed by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC). The MCB binding to GSH is an energy-dependent process since no labelling could be seen at low temperature. When 24-h-old schistosomula were depleted of GSH by buthionine sulfoximine (a specific inhibitor of GSH synthesis) for 18 h, a significant decrease (P<0·001) in fluorescence was observed. PZQ treatment of the schistosomula after first labelling the parasites with MCB did not greatly affect MCB binding to GSH. However, when the 24-h-old schistosomula were first PZQ treated and afterwards labelled with MCB, the pattern of labelling was identical to that of those of the non-labelled parasites. When 24-h-old schistosomula were first PZQ treated, washed and labelled in the presence of 1 mM GSH, the level of fluorescence was recovered. These results suggest that PZQ depletes GSH from schistosomula, and may render them susceptible to the host's immune system.