Evidence is presented to show that the initiation of DNA replication in E. coli 555-7 requires synthesis of a protein whose production is correlated with total protein synthesis. Once replication is initiated, however, reinitiation will occur if all further protein synthesis is prevented; a small amount of protein synthesis is sufficient to prevent this unregulated reinitiation. This shows that the initiation of DNA replication is under negative control. A mechanism for the control of DNA replication is proposed; in this mechanism a replication repressor is synthesized periodically, while an antirepressor protein is synthesized continuously. Derepression of initiation results after sufficient accumulation of the antirepressor protein, and repression is re-established by repressor synthesis after the initiation of replication.