Anti-nucleolin antibodies have been detected in patients with systemic connective tissue diseases (SCTD) including systemic sclerosis (SSc) and systemic lupus erythematosus (SLE). In vivo bound autoantibodies to nucleoli of epidermal keratinocytes have been demonstrated in skin from patients with SCTD. In this study, monoclonal antibody to nucleolin (D-3) was used to determine the distribution of nucleolin in different culture cells including HEp-2, HepG2, HRCC, Molt-4 and Wil2 cells. Nucleolin was found to be present on the surface of HEp-2 and HepG2 cells, but not on the surface of HRCC and lymphoblastoid (Molt-4 and Wil2) cells; in contrast, nucleolin was detected in the nucleoli of all permeabilized cells examined. In immunoprecipitation, using extracts from 32P-labeled HEp-2 cells as antigenic source, cell membrane as well as nuclear nucleolins were found to be phosphorylated with a molecular weight of 105 kDa. Viable HEp-2 and HepG2 cells were cocultured with IgG fraction of D-3 in a CO2 incubator for 1 to 24 h, and then permeabilized with acetone followed by immunofluorescence staining with FITC-labeled goat anti-mouse IgG antibodies. Nucleolar staining was observed in cells after 10 h or longer of coculture. These data indicated that D-3 antibody reacted with cell membrane nucleolin and subsequently gain access into cells in a process related to pinocytosis.