BACKGROUND Exposure of Chinese hamster ovary (CHO) cells to fractionated x irradiation in vitro has led to expression of a distinctive multiple-drug-resistant phenotype. This phenotype is characterized by overexpression of P-glycoprotein without an increase in P-glycoprotein messenger RNA or gene amplification; increased glutathione S-transferase (GST) activity; and resistance to vincristine, colchicine, and etoposide but not to doxorubicin. OBJECTIVE To investigate whether this phenotype is dominant or recessive, we established intraspecific hybrids by fusion of x-ray-treated, drug-resistant CHO cells (DXR-10I or DXR-10II) with drug-sensitive CHO cells (E29). METHODS Drug resistance levels were determined in the wild-type CHO cell line AuxB1, the drug-sensitive E29 line, the x-ray-pretreated lines, and the hybrid lines by colony-forming assay of cells grown in increasing concentrations of colchicine, vincristine, or doxorubicin. The hybrids were characterized by analysis of DNA content, P-glycoprotein expression by Western blotting, GST activity by use of 1-chloro-2,4-dinitrobenzene as substrate, and sensitivity to reversal of resistance to vincristine by exposure to verapamil. RESULTS These hybrids proved resistant to colchicine (two-fold) and vincristine (five- to seven-fold) but not to doxorubicin. After the hybrids were exposed to verapamil, vincristine cytotoxicity was increased 10- to 12-fold. The hybrid lines exhibited levels of P-glycoprotein comparable to those of the unfused x-ray-treated parent cell line, suggesting that P-glycoprotein overexpression is a dominant trait in these hybrid lines. Interpretation of the role of increased GST activity in these hybrids was inconclusive because of the very high levels of GST in the drug-sensitive cell-fusion partner. CONCLUSIONS The multiple-drug-resistant phenotype following x-ray treatment of CHO cells in vitro was dominantly expressed. Overall, these data are consistent with the hypothesis that this phenotype is a consequence of the dominant genetic alteration resulting from exposure to x irradiation. CONCLUSIONS This work adds weight to our hypothesis that there is a biological basis for the expression of clinical drug resistance in certain patients whose tumors have been previously irradiated.