Raji cells are a continuous lymphoblastoid cell line derived from Burkitt's lymphoma (Pulvertaft, 1965) and possess certain B cell characteristics, such as low-avidity IgG Fc receptors and C3-C3b, C3d and Clq receptors, but are devoid of membrane-bound immunoglobulin. These receptors make the cells potentially useful in detecting complement-fixing immune complexes which may be quantitated in pathological sera by reference to a standard curve of aggregated human gammaglobulin (Theofilopoulos, Wilson & Dixon, 1976; Theofilopoulos & Dixon, 1976). In our laboratory we have found that the Raji cell assay (RCA) is frequently positive in sera of patients with systemic lupus erythematosus (SLE) and that there is a significant correlation between anti-nuclear antibodies (ANA) and Raji cell binding. This raises the possibility that the RCA is positive either because these sera contain immune complexes (IC) or because ANA are being detected directly