lambdapolA phages carrying the polA gene in either orientation were isolated and characterised by genetic tests and by assay of the polA gene product after infection of E. coli or induction of lysogens. Lytic infection gave consistently better amplification of DNA polymerase I than that obtained by induction of a lysogen. Optimal amplification of DNA polymerase I was not achieved from the PL promoter of cro-phages, but some advantages accrued when the polA gene was oriented for transcription from the PL promoter of a cro+ phage. lambdapolA phages in which the polA allele was from E. coli strain C600 provided better amplification than phages with the polA allele from E. coli ED8659. Induction of a lambdapolA1 cI857 Qam Sam prophage gave levels of DNA polymerase I approaching 100 times that found in the non-lysogenic Pol+ host. Genetics studies with the lambdapolA phages confirmed the previously postulated orientation of the polA gene within the E. coli genome.