The phenomenon of T cell stimulation by MHC class II expressing (MHC II pos ) CD4 + T cells has been intensively investigated for T cell clones but, so far, not for native T cells. The extensive use of T cell clones may explain the inconsistent outcomes of T cell-mediated antigen-presentation. Therefore, we used freshly isolated primed rat CD4 + T cells induced by immunisation with an allogeneic peptide P1, which is involved in allograft rejection.MHC II pos and MHC II neg CD4 + T cells were isolated from popliteal lymph nodes of P1-immunised Lewis rats and were purified by combining depletion and positive selection steps. Purified MHC II pos CD4 + T cells and MHC II neg CD4 + T cells (10 5 cells per well each) were autostimulated or restimulated with P1-loaded (33μg/ml peptide P1) and subsequently irradiated (with 20Gy) autologous DC.Seven days after immunisation, a small population of MHC II pos CD4 + T cells was detectable (approximately 8.0% of total lymph node cells), as well as a large population of MHC II neg CD4 + T cells (up to 45%). Antigen-specific proliferation was observed for both T cell populations but only P1-loaded MHC II pos CD4 + T cells presented antigen presenting cell (APC) function for P1-primed T cells. Their inability to activate unprimed T cells may be due to impaired surface expression of costimulatory molecules (CD80 and CD86).Immunisation with the allogeneic peptide antigen P1 induced antigen-specific MHC II pos CD4 + rat T cells demonstrating perfect APC function for primed T cells in vitro.