The effect of monoclonal anti-(DNA polymerase alpha) immunoglobulin G (IgG) and F(ab) fragments on DNA replication in lysolecithin-permeabilized human cells and on DNA polymerase alpha activity was determined. DNA polymerase alpha activity in vitro was inhibited equally by the same concentrations of monoclonal IgGs and F(ab) fragments. However, the IgGs and F(ab) fragments were not equally potent in inhibiting DNA replication in permeable cells. In general, the F(ab) fragments were approximately equal to 10-fold more potent than IgGs in inhibiting DNA replication, suggesting the F(ab) fragments cross the nuclear membrane more readily than IgGs. Immunocytochemical studies demonstrated that at least a fraction of anti-(DNA polymerase alpha) IgGs entered the nucleus of permeable cells. For most antibodies tested, the IgG or F(ab) concentration needed to inhibit replication was several orders of magnitude higher than that needed to neutralize polymerase alpha activity extracted from the same number of cells. Anti-(DNA polymerase alpha) F(ab) fragments were shown to inhibit the discontinuous synthesis of Okazaki DNA, as well as the maturation of Okazaki DNA to larger DNA, thereby implicating DNA polymerase alpha in both of these processes.