WE postulate that eukaryotic DNA replication occurs at discrete intranuclear macrostructures, replication complexes, which might be separable from the bulk of nuclear material. A complex composed of DNA, protein and other nuclear constituents could have a different intrinsic density from nucleic acids or protein alone, and this might be the basis for its isolation. Replication complexes would characteristically contain nascent DNA1–4 and DNA polymerase. If they are functionally intact, they should also have the intrinsic capacity to synthesise DNA. We have used a previously reported method for the fractionation of sonically disrupted nuclei on gradients of Cs2SO4 (ref. 5). The results indicate that replication complexes may be partially purified by this method, and that ATP-dependent DNA synthesis is a characteristic of the complexes.