Treatment of the eukaryotic organism Tetrahymena with various types of DNA-damaging agents has been reported to cause a 35-fold induction of a mitochondrial DNA polymerase. We here report that the enzyme can be induced in large-scale cultures by exposure of the cells to thymine starvation and/or intercalating agents. The induced DNA polymerase has been purified to near homogeneity, with a specific activity of approx. 300,000 units/mg protein. The relative molecular mass of the active form of the enzyme is approx. 100,000, as determined by glycerol gradient sedimentation. The subunit structure has been analysed by SDS polyacrylamide gel electrophoresis of the highly purified preparation and by immunoprecipitation with a monoclonal antibody directed to the DNA polymerase. A polypeptide of Mr 47,000 has been observed to be a subunit of the enzyme. This corresponds to the size of the subunits suggested for mitochondrial DNA polymerase from chicken embryos and mouse myeloma cells.