P-glycoprotein 170 encoded by the MDR-1 gene mediates export of substrates including some immunosuppressive drugs. Rapamycin was compared to cyclosporine A for its ability to inhibit P-glycoprotein on normal human peripheral blood mononuclear cells (PBMC). Rhodamine 123 dye efflux measures P-glycoprotein activity and inhibition of P-glycoprotein results in dye retention. Normal CD4+, CD8+ and B cells include a substantial subset with cyclosporine A-sensitive rhodamine efflux. Rh123 dye efflux is also inhibited by rapamycin at comparable drug levels used in transplant models. CsA is approximately 100-fold more effective on inhibition of PBMC P-gp than is RAPA. P-glycoprotein inhibition of ex vivo lymphocytes with three multi-drug resistant T-cell lines showed susceptibility of P-glycoprotein to rapamycin dependent on the cell type. Compared to cyclosporine A, the reduced ability of rapamycin to inhibit P-glycoprotein reflects a reduced avidity in its binding to P-glycoprotein and perhaps increased access to the cell interior. The increased efficiency of RAPA as an immunosuppressive may in part be a result of its relatively low avidity for P-glycoprotein. The authors speculate that interactions with P-glycoprotein may partially modulate the immunosuppressive effects of rapamycin.