The immune system of swine is unique in that the expression of CD4 and CD8 antigens defines four subpopulations of extrathymic resting T lymphocytes in blood as well as in lymphoid tissues. Recently, we documented an additional peculiarity of the porcine T lymphocyte population: a substantial proportion of resting T lymphocytes show constitutive surface expression of MHC II antigens (Saalmüller et al. 1991, Immunobiol. 183, 102). These cells belong predominantly either to the CD4-CD8+ or to the CD4+CD8+ T lymphocyte subpopulations. This report describes the antigen-presenting cell function of MHC II+ T lymphocytes in mixed leukocyte cultures. Experiments employing highly purified T lymphocytes as alloantigen-presenting stimulator cells resulted in a high proliferative and cytolytic alloantigen-specific T lymphocyte response. After separation of T lymphocyte stimulators into MHC II- and MHC II+ fractions, alloantigen-presenting cell function could be assigned to the MHC II+ T lymphocyte subset. Although the mechanisms involved in the response to alloantigen are not completely understood and may be different from conventional antigen presentation, this is the first report where naive resting MHC II+CD8+ T lymphocytes can be defined as antigen-presenting cells, presenting alloantigen to cells of their own lineage.