Studies that made use of multiple assay systems demonstrated increased levels of immune complexes (IC) in patients with systemic lupus erythematosus (SLE), but no consistent correlations of IC concentrations to patterns or activity of disease have been observed. Furthermore, consistent associations between qualitative differences in IC and disease manifestations have been elusive. IC interaction with erythrocytes and mononuclear phagocytic cells is another variable in SLE that may also mediate some of the biological effects of IC. The present report concerns studies of the composition of purified IC obtained from individuals with SLE and other rheumatic diseases; a 64,000 dalton component identified as the A-B subunit of Clq was detected in purified IC from 27 of 51 SLE patients (53%). The presence of this 64,000 dalton component was not related to either IC concentration or to the serum Clq level. However, the presence of the Clq component in isolated SLE IC did correlate with the presence of renal disease (p < 0.02). These observations are interpreted relative to a recently described kinetic model of IC clearance.