The possibility that cerebral lysosomes in the white matter from patients with multiple sclerosis (MS) are more fragile than in normal controls was investigated in tissue obtained at necropsy. In the MS cases no differences were found in results from samples obtained from histologically distinct areas of white matter, i.e. plaque, periplaque and macroscopically normal white matter. However, in comparison to white matter from a group of controls, certain differences were found. Firstly, the cerebral lysosomes in MS white matter appeared to be more fragile than those in the control cases. Secondly, the remaining intact lysosomes in MS cases showed different characteristics from those obtained in control cases. These results, in the light of earlier experiments in rat brain, were interpreted as further evidence of a greater fragility in the cerebral lysosomes from MS cases. The implications of these results in relation to current thinking on the aetiology of MS are discussed.