Rat cerebral cortex neuronal nuclei were isolated by a mild technique utilizing sucrose; citric acid was not used in the isolation of the nuclei. These nuclei in 0.0145 M NaC1, 4.5 per cent sorbitol, and 0.6 mM NaHCO3 with pH 7.2 plus or minus 0.1 at 25 degrees C had an electrophoretic morbidity of minus 2.01 mum-s(-1)-V(-1)-cm(-1). The mobility curves for the brain nuclei indicated that the surface had an acid-dissociable group of pK APPROXIMATELY 2.7. Nuclei treated with 50 mg neurominidase/mg particle protein had a mobility of minus 1.4 mum-s(-1)-V(-1)-cm(-1). DNase or RNase at 50 mug/mg protein had no effect on the mobility of the isolated nuclei. Concanavalin A at 50 mug/mg protein decreased the nuclei electrophoretic mobility to minus 1.82 mum-s(-1)-V(-1)-cm(-1). The results are interpreted to mean that the brain nuclear external surface contains terminal sialic acid residues, but it is completely devoid of nucleic acids, and it binds canavalin A.