Gangliosides that possess the same sugar sequence but differing in the type of sialic acid (N-acetyl- or N-glycolylneuraminic acid) were compared for their reactivity with Hanganutziu-Deicher ('serum sickness') antibodies by double-diffusion gel precipitation tests. Only N-glycolylneuraminic acid containing gangliosides formed precipitation lines with Hanganutziu-Deicher antibodies, thus suggesting that Hanganutziu-Deicher antigens are gangliosides that contain N-glycolylneuraminic acid.