The effects of 2-cyclohexylcarbonyl-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinolin-4-one (praziquantel, EMBAY 8440, Biltricide) on Schistosoma mansoni was studied in vivo and in vitro by means of electron microscopy. It was found that 1. worms were contracted and became dislodged to the liver; 2. intensive vacuolisation occurred at distinct sites of the tegument within 5 min; 3. the surface coat remained unaffected; 4. host cells of the defense system attached to the tegumental vacuoles and started to penetrate into the interior of the parasites about 4 h after treatment; 5. only oral treatment of mice with a curative dose was lethal for the parasites, whereas worms treated with a subcurative dose recovered from the phenomena described above; 6. worms were fixed by fibroblasts to the walls of blood vessels 17 h after treatment; 7. the interior of the parasites was destroyed by host cells - especially eosinophilic granulocytes - while their exterior structures remained unchanged for some time; 8. worms disintegrated completely in the centre of typical granulomas as seen on days 14 and 18 after treatment; at this time only residues of the parasites were still recognisable within necrotic zones.