DNA Polymerase-alpha/primase complexes have been isolated from human neuroblastoma IMR-32, embryonic chicken brains (ECB) and rat prostate tumor PA-3 cells. In the presence of (NH4)2SO4 the major part (90%) of primase activity is released from the Pol-alpha/primase complex. A novel hydrophobic interaction column was used for purification of the primase from PA-3 cells. A nuclear protein factor (NPF-1) that stimulates DNA pol-alpha/primase activity has been purified from rat liver of various ages (3-6 months). The nuclear protein factor which only stimulates the primase activity is under investigation. The monoclonal antibodies (SJK 132-20 and 237-71) were used to detect DNA pol-alpha polypeptides from 11- to 19-day-old embryonic chicken brains.