In the present study, six multiple sclerosis (MS) brain white matter biopsies were analyzed both morphologically and chemically. The purpose of this study was to test the reliability of the earlier postmortem observations. Special attention was given to proteins which were analyzed in polyacrylamide disc gel electrophoresis and to acid proteinase. The results show that myelin basic protein (BP) was present in all biopsies, although advanced demyelination was seen in one case as based on morphology. This finding is controversial to earlier observations. In addition to this, some acid proteins were decreased or lost in two cases. The activity of acid proteinases was increased from two- to fivefold in four of six cases. This increase of the activity was seen also in the normal-appearing MS autopsy white matter which served as control. The source of this activity is discussed. On the basis of the present results it is likely that the role of the BP in the breakdown of myelin in MS and the role of acid proteinases has to be studied in more detail.