We have constructed a pBR322 plasmid derivative which expresses dnaA protein under the control of the E. coli lac UV5 promotor. Expression of the dnaA protein from the plasmid is inducible by isopropyl-β-D-thiogalactoside. In a dnaA⁺ strain induction has no effect on the accumulation of DNA. In contrast, in a thermosensitive dnaA46 strain, induction, at either the permissive or the nonpermissive temperature, results in an immediate stimulation of DNA accumulation. We conclude that, while in a dnaA46 strain dnaA protein limits DNA replication, in a dnaA⁺ strain dnaA protein activity does not control the timing of replication initiation.