DL-alpha-difluoromethylornithine (DFMO), 4-deoxypyridoxine (4-DOP), and methylglyoxal bis (guanylhydrazone) (MGBG) were tested as inhibitors of acute skin graft rejection. Proximal full thickness tail skins were exchanged between C57BL/6 and Balb/C mice. Distal autografts were placed to monitor healing. Inhibitors were given singly or in combination, either orally or by injection, in various schedules to 10 groups of mice. Compared to controls, singly treated mice had significant mean prolongation of allografts ranging from 126% to 141%. Likewise, DFMO plus MGBG extended mean time of complete rejection ranging from 172% to 206%. Autografts remained intact. Some grafts persisted after discontinued immunosuppression. Complete rejection was preceded by a decline in vascularity of the graft bed and/or gradual replacement by host tissue. Graft protection in such stringent circumstances i.e., the use of skin in strains with complete histoincompatibility at the H-2 MHC loci, clearly indicate the anti-rejection effects of polyamine synthesis inhibitors. Moreover, primary and secondary effects of DFMO establish the critical role of polyamine pathway activation in acute rejection. In doses and schedules used, toxicity was encountered when DFMO and 4-DOP were used in combination and when increased amounts of MGBG were administered.