Genetic characterizations of the Chinese hamster ovary cell mutants resistant to the DNA polymerase inhibitors (aphidicolin, ara-A and ara-C) have been described. Resistance to all three inhibitors showed dominance among the progeny of somatic cell crosses between the wild type and mutant parents. Analysis of the segregation of the drug-resistant character among 566 hybrid progeny from somatic crosses between the wild type (aphs, ara-As, and ara-Cs) and the triple mutants (aphr, ara-Ar, ara-Cr) showed the involvement of at least three unlinked genes in controlling the expression of the resistance to different DNA polymerase inhibitors. The mutant (aphr) DNA was used to transfect aphidicolin resistance to recipient mouse Ltk- cells either directly or in combination with the plasmid pTK2 DNA. The aphidicolin resistance of the transfected cells was found to be a stable phenotype and could be used in multiple rounds of transfection, indicating the chromosomal integration of the transfecting gene.