N-Terminally truncated DNA polymerase from Thermus thermophilus (delta Tth polymerase) lacking 5'-3' exonuclease activity was used for DNA sequencing and polymerase chain reaction (PCR). In contrast to the high background of the sequencing ladder observed with the wild-type Tth polymerase, delta Tth polymerase gave readable sequencing patterns which extend up to more than 500 bases from the primer site on cycle sequencing and automated sequencing. The delta Tth polymerase was used for the standard and mutagenic PCR, and net amplification of the DNA and the mutations accumulated during PCR were analyzed. Under mutagenic PCR, the mutation rates were 7.0 x 10(-4) (Tth) and 8.3 x 10(-4) (delta Tth) per nucleotide per cycle of amplification, which were 4-9 times higher than the rates under standard PCR.