Sera from patients with systemic lupus erythematosus (SLE) have been tested for antibody to a human neuronal cell line, SK-N-SH, derived from a metastatic neuroblastoma. With a complement-dependent 51 Cr-release cytotoxicity assay, 75% of SLE sera had antineuronal activity mediated by IgM antibody. Most of the sera containing this IgM neurocytotoxic antibody were also cytotoxic to the human glial cell lines A-172 and U-118MG. The sera did not mediate complement-dependent 51 Cr release when tested against normal human fibroblasts or peripheral blood lymphocytes. IgG antineuronal antibody was detected in 17% of SLE sera by an antibody-dependent, cell-mediated cytotoxicity assay with SK-N-SH cells as targets. The relationship of IgM and IgG antineuronal antibodies to the antilymphocyte antibodies present in SLE sera was evaluated by a series of crossabsorption experiments using SK-N-SH cells to remove neuronal antibodies and WI-L2 (human lymphoblasts) to remove antilymphocyte antibodies. Most of the complement-dependent neurocytotoxicity was not removed by multiple lymphoblast absorptions, although the WI-L2 cells readily removed lymphocytotoxic activity as assayed on normal lymphocytes. Absorption with SK-N-SH cells removed most, but not all, of the lymphocytotoxic antibody. Thus, although lymphocytotoxic antibodies reactive with membrane antigens shared by lymphocytes and brain may constitute a subset of the antibodies to neural cells, most of the antineuronal activity in SLE serum is directed at other cell surface antigens expressed on neuronal and glial cells. Should they gain access to the brain, these antibodies have the potential to produce neuropathology, but their presence in the nervous system of patients with the neuropsychiatric manifestations of SLE is yet to be documented.