The mutagenesis- and repair-enhancing plasmids pKM101 and R205 were introduced into a series of Esherichia coli K-12 polA mutants including two temperature-sensitive mutants. Polymerase levels in extracts of these strains were assayed using an activated DNA template. In none of the cases did the presence of the plasmid in the strains change either the initial rate of incorporation of [3H]thymidine triphosphate into acid-soluble material or the subsequent degradation of the template at longer reaction times. Neither did the presence of the plasmids affect the proportion of N-ethylmaleimide-sensitive polymerase activity detected. Previous studies have reported increased polymerase I-like activity of polA mutants of Salmonella typhimurium and Pseudomonas aeruginosa upon introduction of mutagenesis- and repair-enhancing plasmids. Our experiments indicate that, at least, such an increase in polymerase-I-like activity is not an obligatory phenotype associated with these plasmids.