In order to assess the suspected influence of indwelling lines on the measurement of cyclosporine (CSA) blood levels, we evaluated CSA concentrations in samples drawn simultaneously from both silicone catheter lumens and peripheral veins of eight bone marrow transplant recipients. Blood samples were obtained at 30 min or 6–12 h after discontinuation of intravenous CSA. CSA concentrations were determined in whole blood by specific monoclonal antibody assay. Mean CSA levels from the lumen used for CSA administration were significantly higher than those from samples drawn from the second lumen (i.e., not used for CSA infusion) or peripheral vein. This difference, obtained when the infusion had been interrupted for 30 min, was still the case 6–12 h after discontinuation of CSA infusion. No difference was found between samples obtained from the second lumen and peripheral vein. These observations suggest that CSA might be adsorbed or bound by the silicon catheter and, thus, CSA blood level measurements may be misleading. The second lumen of the catheter or a peripheral vein should be used for accurate monitoring of CSA concentrations.