A monoclonal antibody against purified calf DNA polymerase alpha (deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, EC 2.7.7.7) was used to immunoprecipitate proteins from a crude soluble extract of growing monkey BSC-1 cells. Immunoprecipitates contained familiar DNA polymerase alpha catalytic polypeptides of Mrs approximately equal to 115,000 and 70,000 and also a Mr 40,000 catalytic polypeptide; the major component in the immunoprecipitates, however, was a polypeptide of Mr approximately equal to 190,000 not previously identified as a DNA polymerase. This protein was capable of DNA polymerase activity after electroelution from NaDodSO4/polyacrylamide gels and renaturation. The highly purified enzyme so obtained was active with poly(dT).oligo(rA) as template.primer, resistant to dideoxy TTP (ddTTP), and inhibited by aphidicolin and butylphenyldeoxyguanosine 5'-triphosphate, thus identifying it as a DNA polymerase alpha. The results indicate that a polypeptide of Mr approximately equal to 190,000 is an abundant component among DNA polymerase alpha catalytic polypeptides in growing monkey cells.