Initiation of in vitro phage phi29 DNA replication requires the formation of a heterodimer between a free molecule of terminal protein (TP), which acts as primer, and the viral DNA polymerase. We have analyzed membrane vesicles from phi29-infected Bacillus subtilis cells by quantitative immunoblot techniques. During phage DNA synthesis, large amounts of the viral proteins p1 and free TP were recovered in membrane fractions, as well as a low percentage of the total viral DNA polymerase. Interestingly, the amount of DNA polymerase in membrane fractions increased when viral DNA replication was blocked. Both protein p1 and free TP showed affinity for membranes in the absence of viral DNA. The association of protein p1 with membranes was abolished when the C-terminal 43 amino acid residues were deleted. The above results, together with the critical role of protein p1 for in vivo phi29 DNA replication, led us to conclude that a preliminary stage in the initiation of in vivo phi29 DNA replication could be the assembly of a membrane-associated multiprotein complex containing at least protein p1, free TP and DNA polymerase. Membrane-attachment of this complex could be directly mediated by both protein p1 and free TP. The ability of free TP to bind to membranes and to prime phi29 DNA replication would enable a nascent viral DNA molecule to become membrane-associated when its synthesis begins. We postulate that a general function of the TPs covalently linked to linear DNA genomes in prokaryotes might be, in addition to act as primer, to anchor the linear DNA molecule to the bacterial membrane.