Six synthetic peptides represented as variable sequences of the MHC class 1 molecule H-2Kb were synthesized. Appropriate conditions were elaborated for induction of specific suppressor T cells in vivo by peptide 6 (alpha 2 domain) that is introduced intravenously in a dose of 33 micrograms into the tail vein or 100 micrograms into the orbital venous sinus with subsequent testing for inhibition of proliferation in vitro in response to irradiated stimulator cells Kb (BL/6), whereas the stimulator Kk (B10.BR) did not induce any suppressor activity. Of six different peptides tested, suppressor T cells were induced efficiently by peptides 2 (domain alpha 1), 5, and 6 (domain alpha 2), while the highest suppressor efficiency was realized by i/v injection of peptide 2 into mice preimmunized with EL-4 cells. In the same conditions in vivo, immunization by peptides did not induce any cytotoxic T lymphocytes (CTL). In contrast, specific CTL of high efficiency were induced when memory cells were incubated in vitro with heated BL/6 stimulator cells for 4 days. Intravenous injections of peptide 6 into mice gave rise to prolongation of (BL/6 or B10.MBR) skin graft retention Kb when transferred into allogeneic R101 and B10.AKM mice, respectively, but had no influence on rejection of skin grafts from foreign donors of Kk (B10.BR) or Kd (DBA/2) haplotype, respectively.