Multidrug resistance (MDR) in human cancer cells is multifactorial. Previously, we reported on the association between expression of P-glycoprotein (Pgp), the multidrug resistance-associated protein (MRP), and the lung resistance protein (LRP) with the MDR phenotype in the NCI panel of 60 human cancer cell lines used for in vitro anticancer drug screening. Eight cell lines from this panel, manifesting widely divergent levels of in vitro drug resistance were chosen to investigate the role of MRP and LRP expression at the molecular level. LRP mRNA levels, as determined by ribonuclease protection assay, varied significantly among the 8 cell lines, and correlated closely with in vitro drug resistance to both MDR and non-MDR related drugs. LRP mRNA expression was determined to be a stronger correlate of drug sensitivity than protein expression. In contrast, MRP mRNA levels were not significantly correlated with drug sensitivity. The rates of newly transcribed LRP or MRP mRNA did not correlate with mRNA levels, indicating that mRNA stability or other features of processing may be important in regulation of LRP and MRP mRNA levels. Using Southern blot analysis, LRP gene amplification was shown not to be associated with LRP overexpression. These data suggest that LRP expression may be an important determinant of the MDR phenotype in cell lines intrinsically resistant to cancer chemotherapeutic agents.