Six X-ray-sensitive strains (xrs) of the Chinese hamster ovary (CHO) cell line, all of which have a defect in double-strand break (dsb) rejoining, have been investigated for their proficiency in DNA transfection assays. All 6 strains and clonal isolates derived from them, show a decreased stable transfection frequency using the plasmids pSV2neo and pSV2gpt after transfection by either the CaPh method or the polybrene method. The magnitude of this effect is DNA concentration dependent and is more marked after transfection with higher DNA concentrations (5-20 micrograms DNA). A spontaneous X-ray-resistant reactivant (or revertant) of one xrs strain also acquired the elevated transfection frequency of the wild-type strain providing evidence for a causal relationship between the decreased transfection frequency and the xrs phenotype. In contrast, the strains show no defect when transfection is assayed using a transient transfection system. Since the transient transfection assay only depends on the uptake and transcriptional activity of foreign DNA, and does not necessitate DNA integration, this suggests that the xrs strains do not have a defect in the uptake of foreign DNA, but might have a defect in integration or the processing of DNA molecules prior to integration.