IMMUNOGLOBULIN M, γM, is not detectable by immunoprecipitation techniques in normal cerebrospinal fluid (CSF).1In early studies, this immunoglobulin was shown to be a macroglobulin with a sedimentation coefficient of 19S.2,3In recent studies,3-13however, smaller proteins with antigenic determinants identical with the heavy chain (μ) of 19S-γM (HMW-γM) have been demonstrated. These lower molecular weight forms of γM (LMW-γM) occur in patients with a variety of disorders including systemic lupus erythematosus (SLE),6,7ataxia-telangiectasia,7antibody deficincy syndromes,5,7,12,13, malignancy of the plasma-cell-lymphatic system,10and as an M protein in a lymphomatous disorder.9Some LMW-γM proteins have been shown to have antinuclear and incomplete isohemagglutinin activity.7To date, LMW-γM has not been demonstrated in the CSF. This report presents a case characterized clinically by recurrent central nervous system (CNS) symptoms and skin findings typical of SLE, in whom LMW-γM was found in