Randomly and synchronously growing HeLa cells were tested for poly(ADP-ribose) by direct and indirect immunofluorescent antibody techniques. Fluorescence ofpoly(ADPribose) was seen only in the nuclei of intact cells when the direct immunofluorescent antibody technique was used but in both the nuclei and cytoplasm when the indirect immunofluorescent antibody technique was used; fluorescence in the cytoplasm was nonspecific. When It has been suggested that poly(ADP-ribose), a unique biopolymer synthesized from NAD' by a nuclear enzyme(s) (1), is related to DNA polymerase activity in nuclei (2, 3), DNA repair (4, 5), and some regulatory processes in gene expressions (2, 3). Chemical identification ofnaturally occurring poly(ADP-ribose) has been reported (6-9). In 1974, we obtained a specific antibody to poly(ADP-ribose) (10), and this prompted several groups to measure the in vivo content of poly(ADP-ribose) in different experimental systems (11)(12)(13)(14). The presence of naturally occurring antibodies to poly(ADP-ribose) in patients who have systemic lupus erythematosus indicates that poly(ADPribose) is also present in humans (15, 16). As the poly(ADP-ribose) polymerase activity of HeLa cells has been studied extensively (11, 17), we chose these cells for studies on fluorescent antibody staining ofpoly(ADP-ribose). In this work, the following results were obtained by incubation of randomly or synchronously growing HeLa cells with the antibody: (i) poly(ADPribose) synthesis from endogenous NAD', even in acetonefixed cells, occurred during incubation with antibody; (ii) poly(ADP-ribose) synthesis was enhanced by pretreatment of the cells with DNase I, which causes a 4-to 6-fold stimulation of poly(ADP-ribose) polymerase in chromatin; (iii) poly(ADPribose) synthesis was inhibited by 3-aminobenzamide, a potent inhibitor of poly(ADP-ribose) polymerase; and (iv) the fluorescence of chromosomes of HeLa After incubation, the coverslips were rinsed four times with phosphate-buffered saline (10 mM phosphate/0. 14 M NaCl, pH 7.4) and once with distilled water and dried in air. They were subjected to fluorescent antibody staining as described below. Antisera and Fluorescent Antibody. Antisera against poly(ADP-ribose) were raised in rabbits as described (10). Antisera were absorbed with methylated bovine serum albumin, which was used as a carrier for antibody production. Crude gamma globulin was obtained from antisera diluted twice with phosphate-buffered saline by two precipitations with 50% saturated ammonium sulfate. It was dialyzed against 2.7 mM Tris/ 1 mM phosphate buffer (pH 7.8) and then applied to a column of DE52 (Whatman) that had been equilibrated with the same buffer. On stepwise elution with 80 mM Tris/30 mM phosphate buffer (pH 8.0), two bands of IgG were obtained. Both had strong poly(ADP-ribose) binding activity, but the one that eluted first, designated as IgGlIa, had greater antigen-binding activity than the one that eluted second, designated as IgGlIb.