Activities toward arachidonyl-labelled phospholipase A2 substrates were assayed in fractions of white matter and cerebral cortex from control subjects and in fractions of demyelinated plaque, normal-appearing white matter and cerebral cortex from subjects who died with multiple sclerosis. Membranous activity at pH 8.6 in the presence of Ca2+, characteristic of 14 kDa "secretory" phospholipase A2, in either multiple sclerosis white matter or cortex did not differ from controls, whereas membranous activity at pH 4.5 in the absence of added Ca2+, characteristic of lysosomal enzymes was increased over controls in both plaque and normal-appearing white matter but not cerebral cortex. Activity in the cytosol fraction, at pH 8.6 in the presence of Ca2+ and glycerol characteristic of the "cytosolic" 85 kDa enzyme was decreased by greater than 50% in both white matter and cortex samples from multiple sclerosis subjects. Immuno-precipitation and -blotting confirmed that the deficient activity was largely attributable to the 85 kDa enzyme although the enzyme protein was not similarly reduced.