A DNA-nuclear membrane complex has been isolated by two different methods from the nuclei of cultured mouse fibroblast (3T3) cells. One method, utilizing the detergent sarkosyl (sodium lauroyl sarkosinate), yields a DNA-nuclear membrane complex (the M band), which contains virtually all of the DNA in the nuclei. However, treatment of the M band by sonication, vortexing, or freeze-thaw reduces the amount of DNA in the complex by approximately 50-80%, depending upon the phase of the cell cycle from which the complex was extracted. The remaining DNA is tightly bound to the nuclear membrane and resists further shearing procedures. Over 90% of the choline-labeled phospholipid present in nuclei is also found in these sheared M bands. The percentage of DNA associated with the nuclear membrane varies during the cell cycle and correlates well with the onset, continuation, and cessation of DNA synthesis. Thus, although DNA-membrane complexes can be detected throughout the cell cycle, the percentage of DNA bound to membrane increases during late G1 and S and decreases during G2. In addition, there are distinct qualitative differences in the type of DNA present in the membrane fraction, with a more highly d(A-T) rich DNA being present in confluent (G0) cells than in cells during the S phase. This d(A-T) rich DNA may be related to the mouse satellite DNA identified by others. The M band can be separated into two DNA-nuclear membrane subfractions by centrifugation through a continuous sucrose gradient. The relative proportions of these two subfractions depend upon the percentage of sarkosyl present in the M band prior to centrifugation, with complete removal of sarkosyl resulting in a very large increase in the sedimentation velocity of the complex and in the formation of only one fraction. Evidence that this is a complex of DNA with membrane is given by the finding that DNA is dissociated from the complex with Pronase, deoxycholate, or high levels of sarkosyl. Removal of virtually all of the DNA with DNase from this rapidly sedimenting complex does not dissociate any of the phospholipid which still sediments rapidly as a single band. A second method, which yields a DNA-membrane fraction from nuclei, utilizes sedimentation of lysed nuclei to equilibrium in CsCl density gradients. This low-density CsCl fraction contains only 10-15% of the total DNA, but contains most of the nascent DNA, which may be chased into a membrane-free fraction. The DNA-membrane fraction from CsCl gradients possesses properties in common with the M-band fraction and can be converted into an M band. DNA membrane complexes from sucrose gradients, as well as the crude M-band preparation and a non-membrane-associated DNA fraction from nuclei can synthesize DNA in vitro without the addition of an external DNA template or DNA polymerase. In contrast to the activity in the non-membrane-associated DNA fraction, the membrane-associated polymerase activity is strongly stimulated by adenosine triphosphate and is unaffected by ethidium bromide...