A process for preparing pure monosialoganglioside GM1 in the form of its sodium salt. There is provided a process for the isolation and purification of monosialoganglioside GM1 comprising (a) separation of GM1 from a lipidic mixture containing the monosialoganglioside GM1 as the main ganglioside component by ion exchange column-chromatography using an eluent comprising potassium or caesium ions, (b) recovery of the solute from the eluted solution, (c) diafiltration of an aqueous solution of the recovered solute, and (d) second diafiltration after the addition of 1 M NaCl, and recovering GM1. The purity level of GM1 obtained is higher than 99.0%.