Tests with a plasmid-borne ochre suppressor (sup-812) and a chromosomal amber suppressor (supD501) revealed that one of three mutants of S. typhimurium deficient in DNA polymerase I was an amber mutant. Assays performed on crude extracts established that derivatives of this mutant (designated polA3) carrying ochre and amber suppressors had about 13 to 20% respectively of the enzyme activity found in the wild-type parent. The unsuppressed mutant showed less than 1% of the wild-type level of activity. Other properties of the polA3 mutant that were also partially or in some cases completely reversed by the sup-812 and supD501 suppressors included: u.v. sensitivity, methyl methanesulphonate (MMS) sensitivity, reduced ability to effect host-cell reactivation of u.v.-irradiated or MMS-treated bacteriophages, inability to maintain the (Col El) plasmid, and reduced ability to plate the phage mutant P22 c2 hpi-308. Mapping by P1-mediated transduction showed that all three polA mutations lie between metE and rha on the S. typhimurium chromosome, and that the polA mutation is cotransduced with metE at a frequency of 20% and with rha at a frequency of 8%.