We have studied the nature of P-glycoprotein gene transcripts in multidrug resistant Chinese hamster ovary cell lines selected in vincristine. Using a novel, 35S based, reverse transcription polymerase chain reaction assay, we have shown that there is a considerable increase in the level of pgp2 gene transcripts, with levels rising to a hundred fold those observed in sensitive control cells. The level of pgp1 gene transcripts was raised five fold. The levels of pgp3 gene transcripts were close to the limits of detection by the assay, but were increased in the more highly resistant cell lines. An increase in the size of the P-glycoprotein gene transcript was also observed in some of the resistant cell lines.