DNA-containing immune complexes (IC) are believed to have a central causal role in the glomerulonephritis of systemic lupus erythematosus. Extracellular DNA which provides the antigenic source for these ICs circulates as oligonucleosomes (ON). The in vivo glomerular uptake of radiolabeled ON in rats, as well as its binding by cultured rat mesangial cells, was examined. The data show that the binding of ON to kidney, and specifically glomeruli, was almost fourfold greater than that of purified DNA. Uptake appeared dose-dependent and saturable, while there were no differences in hepatic or splenic uptake. Most of the nucleosomal DNA recovered from glomeruli was TCA-precipitable, and on gel electrophoresis was about 100 to 300 bp, a size sufficient to allow formation of large ICs. In vitro studies demonstrated that ON are bound by cultured mesangial cells in a dose-dependent and saturable manner, with a dissociation constant of 1.25 x 10-10 M/liter and 750 binding sites per cell. Autoradiography of cell cultures incubated with radiolabeled ON showed deposition along the plasma membrane which was inhibited by excess unlabeled ON. The data show that binding of ON to glomeruli exceeds that of purified DNA and may be mediated by histones. ON bind to mesangial cells in a receptor-mediated fashion. The data support the hypothesis of in situ formation of DNA-containing ICs and suggest a role for the mesangial cell in lupus glomerulonephritis.