This study compared the efficacy of long-term intermittent immunosuppression in preventing the rejection of a limb transplant across the strongest histocompatibility barrier in ACI --> Lewis rats using the conventional immunosuppressive agent cyclosporine-A and the newer immunosuppressive agents FK-506 (tacrolimus) and RS-61443 (mycophenolate mofetil). The recipient animals were immunosuppressed daily for 14 days postoperatively, followed by long-term intermittent, twice-weekly immunosuppression using cyclosporine 25 mg/kg, RS-61443 30 mg/kg, or FK-506 2 mg/kg. All three immunosuppressive agents were able to prolong the rejection of the skin component of a limb transplant compared with nonimmunosuppressed controls. Eight of nine animals receiving cyclosporine immunosuppression showed signs of rejection of the skin component of the limb transplant while continuing to receive long-term immunosuppression and had a mean rejection time of 61.6 days. Seven of 10 animals immunosuppressed with RS-61443 also showed signs of rejection while still receiving immunosuppression, with a mean rejection time of 43.6 days. Nine of 10 animals receiving FK-506 immunosuppression showed no signs of skin rejection, but died of bacterial pneumonia between 273 and 334 days after transplantation, with a mean rejection time of 296.1 days. There was no statistically significant difference between intermittent immunosuppression with cyclosporine and RS-61443, but FK-506 was significantly superior to both cyclosporine and RS-61443. The implication of this study is that FK-506, but not cyclosporine or RS-61443, is probably the only single immunosuppressive agent capable of preventing rejection of the skin component of a composite tissue transplant. Combination immunosuppression with FK-506 and RS-61443, therefore, may be required to allow composite tissue transplantation to become a predictable clinical reality in the future.