A well defined enzyme comples of approximately 5 X 10(6) daltons that contains phage and host cell components known to be required for the processes of phage transcription and DNA replication has been isolated from bacteriophage T5-infected Escherichia coli cells. In addition to the RNA polymerase of the host cell, the complex contains the phage-encoded: gpC2 which has been implicated genetically as a controlling element of late transcription; gpD9, the DNA polymerase required for T5 DNA replication; the proteins gpD5 (DNA-binding protein), and gpD15 (nuclease) which are both known to be essential for T5 DNA replication and for the initiation of late transcription. The viral gpD5 derived from the purified complex is a phosphoprotein. The enzyme complex also contains, protected from the action of nuclease, double-stranded DNA with an approximate molecular weight of 1 to 2 X 10(6) (2 to 3% of the size of the T5 genome) which is derived preferentially from the center of the T5 DNA molecule. The composition of the enzyme complex suggests that the processes of transcription and replication are integrated in T5-infected cells.