Highly purified DNA polymerase alpha-DNA primase from normal human tissue (human placenta) has been prepared by immunoaffinity purification on immobilized anti-human DNA polymerase alpha monoclonal antibody SJK 287-38. According to data from SDS electrophoresis this preparation consists of subunits of 180, 160, 145, 140 kDa (a cluster of DNA-polymerizing subunits), 73 kDa (function unknown) and 59, 52 kDa (corresponding to primase). Three active enzyme forms of 270, 460 and 575 kDa have been revealed using native electrophoresis followed by detection of DNA polymerase activity.