A novel monosialosyl ganglioside was isolated from human brain in a yield of 2.1 nmol of the lipid-bound sialic acid per g of wet weight (0.09 % of total lipid-bound sialic acid). The ganglioside was resistant to the action of Vibrio cholerae neuraminidase. By treatment with N-acetylhexosaminidase, methylation analysis, CrO3 oxidation, and partial acid hydrolysis, the structure of this ganglioside was determined to be; The treatment of the ganglioside with 1 N formic acid at 80°C for 2 h produced mainly gangliotetraosyl ceramide, suggesting that the terminal N-acetylgalactosamine was more labile on formic acid treatment than galactose at nonreducing terminal.