Normal human cardiac myocytes failed to induce proliferative responses of peripheral blood mononuclear cells from major histocompatibility complex (MHC)-disparate individuals. Induction of cell-surface MHC antigens on such myocytes before culture still failed to induce alloproliferative responses. Similar data were obtained with cocultures of a human cardiac myocyte (W1) cell line and allogeneic lymphocytes. Addition of interleukin (IL)-1, IL-2, or IL-1 plus IL-2 failed to reconstitute the alloproliferative response. Of interest was the observation that preincubation of primary cultures of myocytes or the W1 cell line (especially after pretreatment with interferon-gamma) with an autologous enriched population of adherent cells, containing monocytes/macrophages/B cells, markedly augmented the ability of these cells to induce alloproliferative responses. In addition, the specificity of the secondary response of T cells primed against myocytes or the W1 cell line (especially after pretreatment of the myocytes or the W1 cell line with interferon-gamma) appeared to reside at the MHC type of the adherent cells used in the primary phase. These data suggest that adherent cells most likely present donor myocyte-specific MHC or other proteins to allogeneic T cells in the context of the MHC molecules of the adherent cells. Such a finding provides evidence that the indirect pathway of host cellular sensitization may occur in vivo and play a role in chronic graft rejection.