The invention relates to an isolated cytotoxic factor which is associated with multiple sclerosis and which is selected from the heterocomplex GM2AP/GM2/MRP14 and mutated GM2AP/GM2/MRP14, and to the method of detecting said factor in a biological sample to be tested. The inventive method comprises the following steps consisting in: (i) bringing the biological sample into contact with at least one capture antibody selected from antibodies that bind specifically to the GM2AP protein, to the mutated GM2AP protein, to the MRP14 protein, to the complex GM2AP/GM2, to the complex mutated GM2AP/GM2 and to the complex MRP14/GM2, and with at least one labeled detection antibody selected from antibodies that bind specifically to the GM2AP protein, to the mutated GM2AP protein, to the MRP14 protein, to the complex GM2AP/GM2, to the complex mutated GM2AP/GM2 and to the complex MRP14/GM2, and (ii) detecting and/or quantifying the cytotoxic factor by detecting and/or quantifying the labeled detection antibody.