The distribution of polysialosyl glycoconjugates defined by a monoclonal antibody M6704 (M6704 antigen) and of its O-acetylated counterpart (O-Ac-M6704 antigen) was investigated in human gliomas and in normal adult and fetal central nervous system. Among 32 gliomas, M6704 antigen was confined to the glioma cell surface and cytoplasmic processes in 21 cases, whereas O-Ac-M6704 antigen was expressed in the Golgi region in 22 cases and at the cell surface in seven cases. In adult tissues M6704 antigen was barely detectable, whereas in fetal tissues it was spatially and temporally expressed at the cell surface and along processes of post-mitotic neurons. The O-Ac-M6704 antigen first appeared in the Golgi regions of neurons at 66 gestational days and increased gradually with further maturation to adult levels. Several polysialoglycoproteins, the most dominant being 51 KD, and C-series polysialogangliosides were isolated from gliomas. These results indicate that M6704 antigen is a distinctive component expressed selectively by embryonic neurons during development and by glioma cells.