The present study demonstrated that tacrolimus (FK506) induced apoptosis in mitogen-activated human PBMCs and that several interleukins (ILs) prevented the apoptosis. Mitogen-activated PBMCs obtained from healthy subjects treated with 1 ng/ml of FK506 exhibited a DNA-ladder structure in agarose-gel electrophoresis. Moreover, in quantitative analysis of fragmented DNA using cell death detection ELISA, 1 ng/ml of FK506 induced maximal apoptotic induction, whereas > 10 ng/ml of glucocorticoids (GCs) or cyclosporine (CsA) was required to achieve maximal induction of apoptosis in PBMCs. FK506-induced apoptosis was efficiently attenuated by co-addition of ILs including IL-1beta (2 ng/ml), IL-2 (5 ng/ml) and IL-4 (40 ng/ml) into culture. The results showed that FK506 induces apoptosis in mitogen-activated PBMCs at concentrations which exhibited sufficient suppression of PBMC-blastogenesis. The data also suggested that cytokine networks including those via IL-1beta and IL-4 in addition to IL-2 prevent FK506-induced apoptosis.