The ability of autoantibodies to penetrate living cells is controversial. We have identified immunoglobulin G (IgG) antibodies capable of penetrating an epithelial cell line, COLO-16, in five of 36 (14%) antinuclear antibody positive sera from patients with SLE. Thirty minutes following incubation of cells with dilutions of either whole sera, globulin fractions, or F(ab')2 fragments of IgG, approximately 80-90% of cells demonstrated intranuclear IgG by indirect immunofluorescence. Viability of cells prior to assay was > 98% as determined by trypan blue staining and penetration of IgG into the nuclei did not affect viability or DNA synthesis of the cells in short-term culture. Intracellular IgG could not be detected following exposure of the cells to high-titer reference autoantibodies of known specificities (against DNA, Ro, La, Sm, RNP, or ribosomes). Furthermore, absorption of the sera with either DNA or chromatin failed to abolish intranuclear penetration, indicating that the autoantibodies were not directed against DNA receptors or nucleosomes on the cell surface. Antibody uptake was relatively selective for epithelial cell lines, because intranuclear IgG was not detected in cell lines of lymphoid origin exposed to the sera. Two of the five sera immunoprecipitated proteins of molecular weight 88 kD with or without a 68-kD protein from COLO-16 cells labeled with 125I at the cell surface. These findings indicate that a subset of SLE patients have IgG capable of penetrating a cell line of epithelial origin. These antibodies, most likely, bind to cell surface proteins and are translocated into the cell nucleus. Although direct immunofluorescence of a skin biopsy obtained from one of the five patients with "penetrating IgG" also showed intranuclear staining for IgG, the biologic relevance of these findings remains to be determined.