1. Sera from patients with active lupus erythematosus fixed complement with a wide variety of nuclei from different organs and species, with calf thymus nucleoprotein, and in two instances with histone. Isolated calf thymus, salmon sperm, human leukocyte and pneumococcal DNA also fixed complement with many of these sera. Similar reactions were not encountered in a limited control series including normal individuals and other pathological states. 2. Most active L.E. sera fixed complement with both nuclei and DNA in roughly parallel titer. However, exceptions were encountered and one serum reacted strongly with nuclei but failed to react with DNA. Cross-absorption experiments with nuclei and DNA suggested the presence of 2 distinct serum factors. 3. The L.E. factor appeared to be related to the factor responsible for complement fixation with nuclei but distinct from that responsible for DNA fixation. 4. The significance of these findings with respect to antibodies against nuclear constituents is discussed.