We have previously demonstrated that within 24 h of exposure of the CEM/A7R cell line to epirubicin (EPI), MDR1 gene expression is induced. The aim of the current study was to investigate the role of cyclosporin A (CyA) and PSC 833, two biochemical modulators of the classical multidrug-resistant phenotype, in this model. CEM/A7R cells were exposed to EPI in the presence or absence of various concentrations of CyA or PSC 833. MDR1 expression was assessed using Northern blot analysis and quantitated using a phosphorimager. P-glycoprotein (P-gp) expression was analyzed by the determination of MRK16 binding using flow cytometry. P-gp function was measured in an assay of [3H]daunomycin accumulation. The coincubation of CyA or PSC 833 with EPI prevented the increase in MDR1 gene expression induced by EPI alone. This effect of the two modulators was dose dependent. Neither modulator alone had any significant effect on the expression of MDR1. In these experiments, changes in MDR1 expression correlated with changes in P-gp levels (based on MRK16 binding) and P-gp function. Thus, both PSC 833 and CyA appear to prevent the induction of MDR1 gene expression caused by the short-term exposure of CEM/A7R cells to EPI.