Intravenous cyclosporin was administered continuously to 18 consecutive marrow transplant recipients. Blood samples taken from the single lumen central venous line through which the cyclosporin was administered showed invariably high concentrations. Toxic levels were still present after discontinuation of cyclosporin administration for 8 h, while the peripheral blood levels were in the therapeutic range throughout the observation time. These observations may explain some of the reported discrepancies between the levels of cyclosporin and the side-effects when using intravenous cyclosporin. Mean cyclosporin levels remained stable throughout the 4 weeks of continuous intravenous cyclosporin administration, despite a gradual decrease of the mean cyclosporin dose. Dose adaptations were made based on clinical side-effects and cyclosporin concentrations in blood samples from peripheral veins.