Recently, Clayberger et al. demonstrated that ALLOTRAP, small synthetic peptides derived from a conserved region of the alpha 1 helix of certain HLA class I molecules, inhibited human CTL responses in vitro. In rats, ALLOTRAP 07 therapy combined with a subtherapeutic dose of cyclosporine led to the permanent acceptance of heart allografts. In the present study, the effect of ALLOTRAP on the survival of skin allografts in mice was studied. The tail skin of male C57B1/6 (H-2b) mice was grafted on the back of male CBA (H-2k) recipients. In untreated animals, the skin graft was rejected after 11.6 +/- 1.13 days (MST +/- SD). Cyclosporine administered orally for 5 days after transplantation prolonged graft survival to 13.1 +/- 2.13 days. ALLOTRAP 2702 prolonged graft survival to 16.57 +/- 2.15 days when administered orally for five days posttransplantation and to 18.86 +/- 0.38 when administered intraperitoneally until rejection. Thus, ALLOTRAP peptides derived from human MHC class I sequences, in addition to inhibiting human T cell responses in vitro, also prolong allograft survival in rats and mice.