Overexpression of human MDR1 P-glycoprotein [Pgp] is associated with cellular resistance to bulky amphipathic drugs, such as taxol, anthracyclines, vinca alkaloids, and epipodophyllotoxins by actively effluxing drugs from cells. We have found that human MDR1 transfected murine L1210/VMDRC.06 leukemia cells exhibit relatively large amounts of Pgp and high levels of resistance to 6-mercaptopurine [6-MP] and other purine and pyrimidine nucleobase and nucleoside analogs. L1210/VMDRC.06 cells accumulated 6-MP as the nucleotide in vitro at only about one-third of that formed by parental L1210 cells in normal medium; however, under conditions of ATP-depletion, the amount of 6-MP nucleotide formed was essentially the same in both cell lines. The findings support active efflux of 6-MP in L1210 cells, suggesting involvement of Pgp in 6-MP resistance even though it is generally believed that Pgp does not transport such agents. The resistance pattern observed in L1210/VMDRC.06 cells was not duplicated in P388/VMDRC.04 leukemia cells transfected with the same MDR1 cDNA, even though a similar amount of Pgp was present in both cell lines. Immunofluorescent staining of surface membrane Pgp showed that L1210/VMDRC.06 cells contained at least three-fold more surface Pgp than P388/VMDRC.04, implying that P388/VMDRC.04 cells are unable to actively efflux 6-MP and other antimetabolites as effectively as L1210/VMDRC.06, because of significantly lower membrane Pgp. The findings suggest that the exceedingly large concentration of overexpressed Pgp in the surface membrane of L1210/MDRC.06 cells is responsible for resistance to 6-MP and other purine and pyrimidine analogs, even though these agents usually are not considered to be substrates for Pgp.