Disclosed are uses of Gangliosides G.sub.M1 and/or asialo-G.sub.M1 substances simulating the carbohydrate portion of said gangliosides with regard to bonding to anti-G.sub.M1 antibodies and/or anti-AD.sub.M1antibodies for producing agents which bind or block anti-G.sub.M1 antibodies and/or anti-AG.sub.M1 antibodies which bond to natural killer cells (NKC) or for blocking antigen-presenting cells and producing a T-cell anergy, and for producing an affinity material for the extracorporeal removal of anti-G.sub.M1 antibodies and/or anti-AG.sub.M1 antibodies in order to prevent, inhibit, and treat malignant cancers.