The only function of the transport protein P-glycoprotein (Pgp) that has been identified to date in mammals is its ability to mediate multidrug resistance (MDR) in tumour cell lines. Rodents have three P-glycoprotein (pgp) genes (termed pgp or mdr 1, 2 and 3), and humans have two (mdr1 and mdr3/mdr2). Pgp isoforms differ in their drug transport capabilities: Pgp1 and Pgp2 can mediate MDR, while Pgp3 apparently cannot. The expression of the gene family members is tissue-specific, suggesting that they have unique physiological roles. We report in this paper the complete cDNA sequences for each of the three pgp genes in Chinese hamster. A comparison of the Chinese hamster cDNA sequences with those isolated from human and mouse confirms the identification of the gene family member homologues across these species. An analysis of mammalian Pgp sequences identifies conserved sequences which, it may be speculated, are important for Pgp activity. Previously, three different mdr3 (pgp3 homologous) transcripts, products of alternative splicing, have been reported in humans. Unexpectedly, we find no evidence for a similar alternative splicing event in Chinese hamster: it appears that the expression of pgp3 (mdr3) is different between rodents and humans.