N-acetylneuraminic acid at the surfaces of rat cerebral cortex and liver mitochondria and derived mitoplasts (inner membrane plus matrix particles) was studied biochemically and electrokinetically . Rat cerebral cortex mitochondria in 0 .0145 M NaCl, 4 .5 % sorbitol, pH 7 .2 t 0 .1, 0 .6 mm NaHCO 3, had an electrophoretic mobility of -2 .88 f 0 .01 u/sec per v per cm . In the same solution the electrophoretic mobility of rat liver mitochondria was -2 .01 f 0 .02, of rat liver mitoplasts was -1 .22 f 0 .07, and of rat cerebral cortex mitoplasts -0 .91 ± 0 .04 µ/sec per v per cm . Treatment of these particles with 50 µg neuraminidase/mg particle protein resulted in the following electrophoretic mobilities in µ/sec per v per cm : rat cerebral cortex mitochondria, -2 .27 ; rat liver mitochondria, -1 .40 ; rat cerebral cortex mitoplasts, -0 .78 ; and rat liver mitoplasts, -1 .10 . Rat liver mitochondria, mitoplasts, and outer mitochondrial membranes contained 2 .0, 1 .1, and 4.1 nmoles of sialic acid/mg protein, respectively . 10% of the liver mitochondrial protein and 27 .5% of the sialic acid was solubilized in the mitoplast and outer membrane isolation procedure . Rat cerebral cortex mitochondria, mitoplasts, and outer mitochondrial membranes contained 3 .1, 0 .8, and 6 .2 nmoles sialic acid/mg protein, respectively ; 10% of the brain mitochondrial protein and 49% of the sialic acid was solubilized in the mitoplast and outer membrane isolation solution procedure . Treatment of both the rat liver and cerebral cortex mitochondria with 50 µg neuraminidase (dry weight) /mg protein resulted in the release of about 50% of the available outer membrane sialic acid residues . Treatment of all of the particles with trypsin caused release of sialic acid but did not greatly affect the particle electrophoretic mobility . In each instance, curves of pH vs . electrophoretic mobility indicated that the particle surface contained an acid dissociable group, most likely a carboxyl group of sialic acid with pKa -2 .7 . Treatment of either the rat liver or the cerebral cortex mitochondria with trypsinized concanavalin A did not affect the particle electrophoretic mobility but did cause a decrease in the electrophoretic mobility of L5178Y mouse leukemic cells .