The pleiotropic drug resistance 5 gene (pdr5) encodes a multidrug membrane transporter and plays a very important role in the efflux of a broad range of chemicals in yeast cells. To study the possible function of pdr5 in insect cells, two stably pdr5-transformed lepidopteran insect cell lines, Sf21 and CF-203, were developed. Transcripts of pdr5 were detected in these two lines using Northern blotting and RT-PCR analysis. When cells were treated with the protein synthesis inhibitor diacetoxyscirpenol, the transformed Sf21. and CF-203 cell lines showed increased tolerance to this chemical. However, unlike in yeast cells, ecdysone agonist RH5992 could not be excluded by PDR5, probably because of low expression levels or imperfect incorporation of the recombinant protein in these transformed cell lines.