Abstract Glycogen branching enzymes (EC 220.127.116.11) in extracts of cerebral cortex from a patient with Lafora myoclonus epilepsy and several normal subjects were compared in terms of activity and physical properties of the proteins. Branching-enzyme activity in Lafora cortex tissue was significantly higher than in controls. Gel-exclusion chromatography revealed a single branching-enzyme species of identical elution volume in normal and pathological tissues. A molecular weight of 95,000 was estimated for the enzyme from normal cerebral cortex by means of sucrose density gradient sedimentation. A specific staining method for detection of branching-enzyme activity in polyacrylamide gel slabs was developed for the analysis of electropherograms of tissue extracts. Normal cortex extracts showed the presence of two or more closely spaced bands of branching activity; Lafora cortex extracts showed four strongly stained bands which migrated more rapidly toward the anode than those from the normal tissue. We conclude that Lafora myoclonus epilepsy does not involve a deficiency of total branching-enzyme activity but the protein appears to be of a slightly more acidic nature than that of normal tissue. This may reflect alterations in another system involved in protein processing.