In this study, the effects of the FK506-mPEG on immune cell activity, skin grafting rejection, and Freund's complete adjuvant arthritis were investigated. The proliferation of T cells was inhibited with increase with the FK506 and FK506-mPEG concentrations. FK506 and FK506-mPEG at concentrations between 0.01 nM and 1000 nM had very similar effects on the proliferation of the T cells. On the other hand, in the case of the proliferation of T cells by calcium ionophore A23187 (1 microM), when the FK506-mPEG concentration was increased from 0.01 to 1000 mM, the proliferation was decreased from 90.8 to 40.3%. This was 1.8-fold higher than that of paramethoxyamphetamine (PMA). The inhibitory effect of FK506-mPEG on mast cell proliferation was higher than that of FK506. When B cells were cultured for 7 days in basal medium with no pokeweed mitogen (PWM), the IgG production was 156.2 ng/mL. On the other hand, in the case of the same treatment with 0.25% of PWM, it was 876.4 ng/mL. This is about 5.6-fold higher than with no PWM. These results show that FK506-mPEG may be practically applicable as a prodrug for the immunosuppressant FK506.