Blood level monitoring of FK506 was performed on 6 kidney transplant recipients. Reproducibility was evaluated on trough and AUC levels in both whole blood and plasma samples. Reproducibility was almost the same degree on both trough and AUC levels and in both whole blood and plasma samples. Trough and AUC levels in whole blood and AUC in plasma were low at the onset of rejection episodes, whereas those were high at the onset of adverse effects by FK506 such as hyperglycemia, hyperkalemia and neurological symptoms. Frequent trough level monitoring is useful to detect kidney transplant rejection and adverse effects by FK506.