Primary tissue cultures of human gliomas were treated with cytochalasin B (0.5--60 microgram/ml for 90 min). Cell motility was inhibited irreversibly in glial tumour cells, but the effect was reversible on the mesenchymal cells growing in culture in the lower dose range. Cell adhesion was considerably reduced as the dose was increased, as was the capacity for cells to spread on a surface from suspension. Low concentrations of cytochalasin B caused negligible cell death and little disruption of cell ultrastructure. However, increases in dose were accompanied by a greater predominance of rough endoplasmic reticulum and inclusions and aggregation of microfilament bundles. As seen by scanning electron microscopy, cytochalasin B caused the withdrawal of peripheral cell borders, disappearance of ruffles and the breakdown of cytoplasmic lamellae. Charateristic surface blebs and folds appeared in their place. By comparison, colchicine (1--10 microgram/ml) caused a less marked and non-specific reversible reduction in cell motility on both glial and mesenchymal cells. No significant change in cell adhesion or spreading took place even at high doses, although at all concentrations gross disruption of the cell surface took place with changes in ultrastructure characterised by loss of cytoplasmic microtubules and aggregation of 10 nm filaments.