This study reports the identification of an autoantibody in the sera of some patients with systemic lupus erythematosus that reacted with nuclear antigen(s) of proliferating cells. The autoantibody was initially detected by the observation that it did not react in immunofluorescence with nuclei of renal tubular or glomerular cells, nor with hepatic parenchymal cells, but only reacted with scattered cells in the interstitial tissue of these two organs. In contrast, many lymphocytes in lymph node follicles, spleen, and thymus reacted with this antinuclear antibody. Normal peripheral blood lymphocytes did not show nuclear staining but after mitogenic stimulation, 20% of cells became positive. Nuclear staining was not restricted to lymphocytes but was also observed in several tissue culture cells lines such as Hep-2 cells (human epithelial carcinoma), Ehrlich ascites tumor cells, and baby hamster kidney cells. The reactive nuclear antigen(s) was soluble in physiologic saline and reacted with serum autoantibody to give a precipitin line in immunodiffusion that was immunologically distinct from DNA and other known nuclear antigen-antibody precipitating systems. Autoantibodies to proliferating cell nuclear antigen(s) might serve as useful biologic markers to study stimulated lymphocytes and other proliferating cells.