To study the mechanism of azidothymidine (AZT) cytotoxicity, human DNA was transfected to a variant of Chinese hamster V79 fibroblasts, the tr5 line. This cell line was used for this study for its elevated sensitivity to 5 microM AZT. Primary and secondary transfectants of tr5 cells using total human DNA and pSV2neo plasmid were selected by sequential incubations in AZT (20-50 microM), G418 (400 micrograms/ml active dose), and medium containing hypoxanthine, aminopterin, and thymidine (HAT). One DNA Alu fragment was detected in transfectants using primer TC-65, specific for human Alu sequences in the polymerase chain reaction (PCR). Moreover, cDNA of Chinese hamster alpha-type DNA polymerases was detected in transfectants by reverse transcriptase PCR (RT-PCR) using specific oligo-primer from a DNA polymerase-alpha cDNA sequence and in elevated annealing temperatures. In untransfected tr5 cells, neither of these sequences was detected. The data suggested that the genetic basis for AZT sensitivity may be related to the expression of alpha-type DNA polymerase, and the result indicated that AZT cytotoxicity could be reversed by transfection of appropriate human DNA into tr5 cells. This animal cell model has applications for studies of AZT metabolism and the isolation of the human gene that modulates AZT cytotoxicity.