Cellular resistance to methotrexate (MTX) is believed to be unaffected by expression of MDR1 P-glycoprotein (Pgp), a pleiotropic efflux pump acting on different hydrophobic compounds that enter cells by passive diffusion. A series of human leukemic CCRF-CEM sublines, isolated by multi-step selection for very high resistance to MTX, exhibit multiple mechanisms of MTX resistance, including decreased carrier-mediated uptake of MTX and DHFR gene amplification. These sublines show cross-resistance to drugs of the multi-drug resistance (MDR) family, which is correlated with relative resistance to MTX. The MTX-selected sublines show increased expression and function of the MDR1 gene, based on the measurement of MDR1 mRNA, Pgp and rhodamine 123 accumulation. Sequence analysis of the MDR1 cDNA from MTX-selected CCRF-CEM cells revealed no mutations in the protein coding region. MTX resistance in these cell lines is partially reversible by a Pgp-specific monoclonal antibody (MAb) UIC2 and a monovalent FaB fragment of UIC2. Our results indicate that Pgp can contribute to multifactorial resistance to MTX.