Previous attempts to clone the Escherichia coli polA+ gene onto a high copy number plasmid were unsuccessful. The apparent lethality of unregulated overproduction of DNA polymerase I can be eliminated by cutting at a BglII site 100 nucleotides upstream from the ATG start codon of the polA gene. This permitted the construction of plasmid pMP5 which contains both the coding sequence for DNA polymerase I and the lambda pL promoter for conditional control of polA gene expression. BglII cutting only damages but does not eliminate the polA promoter activity; the BglII site thus lies within the polA promoter region. Leakiness of the damaged polA promoter results in overproduction of DNA polymerase I even under conditions where pL is fully repressed. This overproduction is inhibitory of cell growth, as reflected in both growth rate and in the frequency of appearance of mutant plasmids which are nonproducers of DNA polymerase I. Transformation of plasmid pMP5 into E. coli N4830 yields strain ATL100 which under inducing conditions provides 138-fold amplification of DNA polymerase I. Optimization of growth and expression conditions are presented together with an optimized rapid polymerase purification scheme. In addition to providing a convenient source for preparation of DNA polymerase I, this work serves as the basis for a future detailed molecular genetic analysis of the polA gene product.