The absolute electrophoretic mobility of erythrocytes from MS patients is reduced in the presence of 0.08 mg/ml of linoleic or arachidonic acid, whilst that of normal or other neurological disease patients is increased in the presence of these acids. When an MS patient ingests gamma-linolenate (in capsule form equivalent to 413.4 mg of gamma-linolenic acid and 2.664 g of linoleic acid per day) the reaction of MS erythrocytes begins to change. After 3 or 4 months the reaction becomes normal with arachidonic acid (i.e. mobility is speeded up) and 2 months or so later this occurs also with linoleic acid. Very prolonged administration of gamma-linolenate leads to a markedly increased sensitivity to the effect of prostaglandins (PGE2) on RBC mobility. The observations are interpreted to mean the induction of a biochemical-biophysical change in the membranes, and the significance of this in the aetiology and treatment of multiple sclerosis is discussed.