The effects of the active sulphoxide metabolite of the fasciolicide triclabendazole (Fasinex, Ciba-Geigy) on the vitelline cells of Fasciola hepatica were determined in vitro by transmission electron microscopy using both intact flukes and tissue-slice material. At a triclabendazole concentration of 15 micrograms/ml the vitelline cells of intact flukes showed ultrastructural changes only after prolonged incubation periods (12-24 h). The changes observed were a swelling of the granular endoplasmic reticulum (GER) cisternae with decreased ribosomal covering in the intermediate-type cells and condensation of chromatin and disappearance of the nucleolus in the nucleus of the stem cell. Similar changes were evident more quickly (by 6 h) in whole flukes treated at the higher concentration of 50 micrograms/ml. The shell globule clusters were loosely packed in the intermediate type-2 cells, and the number of intermediate type-1 cells declined with more prolonged incubation. Disruption of the nurse-cell cytoplasm was also observed from 12 h onwards. After only 6 h incubation of tissue-slice material at 50 micrograms/ml, intermediate type-1 cells were absent, shell globule clusters in mature cells were loosely packed and the nurse-cell cytoplasm was badly disrupted. By 12 h the vitelline cells were vacuolated and grossly abnormal. The results are discussed in relation to postulated actions of triclabendazole against the microtubule component of the cytoskeleton and against protein synthesis in the fluke.